Oodles of spare change discovered under the couch probably won't be the most valuable thing you unearth during spring cleaning.
Plenty of valuable items may be collecting dust in junk drawers and closets, from gently worn sweaters to outgrown skis and discarded smartphones—adding up to potentially hundreds of dollars in unclaimed cash.
"It's unbelievable what things are worth," said Mary Hunt, founder of DebtProofLiving.com. She recently rediscovered an old box of Teenage Mutant Ninja Turtles cereal saved for nostalgia and saw similar listings on eBay that have topped $45 in bids.
Even items that seem like junk—used printer cartridges, broken china—may have some value, said Trae Bodge, senior lifestyle editor at RetailMeNot.com. Sites like InkRecycling.org buy the former for a few bucks apiece, and the latter can be sold as craft materials on Etsy. "You would think people aren't interested in those things, but they are," she said.
Sales can take more effort than a fast yard sale, but the payoff can be substantial, particularly in these eight categories:
—By CNBC's Kelli B. Grant
Posted 4 Apri—l 2014
Even a limited spring-cleaning spree should include assessing older model smartphones, laptops, tablets and other electronics. Gadget trade-in has become a competitive market, with programs from retailers (including Amazon and Best Buy), cellphone providers (AT&T and Verizon), manufacturers (Apple, Dell) and independent sites (Gazelle.com, NextWorth.com). Value depends on the item, its condition and whether you have accessories such as the charger and instruction manual. Make your trade in a store or mail it in using a free, prepaid mailer. Most programs promise to wipe your gadget of personal content, but it's still a smart idea to clear it out yourself first. (Look for the reset or wipe option in the settings.)
Some smartphones—notably, the iPhone—may be worth more than you paid for them under contract. A 16GB iPhone 5, which originally sold for $199 with a new two-year cellphone contract, is still worth as much as $224.50 at trade-in, according SellCell.com, an offer comparison site. Even an old flip phone may be worth a dollar or two at resale sites—or more on eBay, if it's a Motorola Razr or another still-popular model.
Read MoreThe value in an old flip phone
Unwanted gift cards could be worth as much as 92.5 percent of their value on the secondary market, according to GiftCardGranny.com, a site that aggregates offers from various resellers. The more popular the retailer, the more you'll get. A card for gas station chain Chevron is worth as much as 91 percent at Junkcard.com, for example, or $22.75 for a $25 card. One for beauty brand Clinique is worth just 52 percent at GiftCardZen.com, or $13 for a $25 card.
Once you have an offer, most sale sites offer free shipping to send in the gift cards; you'll get a check within a week or two. In some cases, you can make the sale electronically by sending the gift card number and PIN, getting paid in a matter of hours.
Haul a bag down to the nearest secondhand bookstore, or try your luck online. Powell's Books and Cash4Books.net (the online arm of McKenzie Books) are among the shops that buy a variety of gently used books; for textbooks, there's also Bookbyte.com and Buyback101.com, among others. On most sites, buyers enter the books' ISBNs to get a quote, and then ship the books for free with a prepaid label. Value varies widely by title, with newer tomes and textbooks more desirable. At Powell's, a copy of "The Fault in Our Stars" nets $3 in store credit or $2 sent to the seller through PayPal.
Don't like the offers you see? Try creating an individual listing on sites such as Amazon or Half.com, which allow sellers to set their own price. If the item sells, you'll get payment equivalent to its sale price plus a credit for shipping, and less any fees. (Half.com takes a cut of up to 15 percent; Amazon, $0.99 per item plus a closing fee of up to $1.35 and a commission of up to 15 percent.)
Old CDs, DVDs and video games can be a tough sell on the secondhand market if you haven't kept the original cases with the artwork, extra materials and UPC code intact. That's a must at sites such as Amazon, Decluttr.com and SecondSpin.com. But for a collection in good condition, shoppers could expect to get a few bucks back per item. At Secondspin.com, a copy of Beyoncé's album "Beyoncé" is worth $5, a DVD of "Kick-Ass 2," $6.75.
As with selling books, most sites offer a quote upfront and free shipping via a prepaid label. Opt for payment by check or online via PayPal. In some cases, a higher value is offered if you take store credit.
Shopping your closet can yield some potentially valuable finds. A growing number of sites and apps, including Poshmark, ThredUP, The Real Real, Tradesy, Twice and MaterialWrld, buy and sell gently used items. Some, including Twice and ThredUP, send a free prepaid mailer and offer an upfront payment for those items. Others let you price and list items yourself, taking a cut in fees or commission. Poshmark, for example, charges a flat fee of $2.95 for sales under $15; on pricier items, the site takes a 20 percent cut.
Value depends on desirability and condition—and sites are often very particular about the items they will accept. At ThredUP, the site estimates a Gap dress will net you $2.70 to $11. The Real Real, which focuses on luxury designer brands, estimates a pair of pants may return $75 to $225, depending on the trendiness and brand.
Outgrown inline skates and forgotten tennis rackets may still have some value, particularly at local sports retailers and consignment groups. Dick's Sporting Goods and Sports Authority both offer a golf club trade-in program, while stores including BassPro and REI offer seasonal trade-in programs on items such as fishing rods and reels, bicycles and kids' snowsport equipment.
Value depends on the desirability and condition of the gear; secondhand equipment chain Play It Again Sports estimates sellers receive between 15 and 25 percent of what the item would sell for, new. SwapMeSports lets users set their own prices and arrange trades with interested buyers. (There, you'll pay for shipping if the buyer isn't local.)
Consumers looking to unload an old lamp or a set of dining chairs aren't limited to local classifieds, yard sales and secondhand shops. Hunters Alley, an offshoot of flash sale site OneKingsLane.com, lets sellers submit listings with photos and prices, for inclusion in the site's boutiques. If the item sells, they'll steer you through the shipping process and send payment for the item, less a commission of 15 percent, plus shipping reimbursement. There's also Chairish, which accepts listings for pieces priced at $250 or more ($150 or more for smaller décor items), with sellers receiving 80 percent of the item's sale price. Sellers in select cities who are looking to unload $1,000 or more in furniture can let the site take on more of the listing legwork, for a 40 percent commission.
The catch: The sites don't accept every item. Chairish, for example, reports most listings are "designer, custom, antique or high-end modern and vintage pieces." Sites also typically require items be in good condition.
Don't schedule that yard sale just yet. In addition to standbys including Craigslist and eBay Classifieds, there are sites such as Junkables and Etsy (for "vintage" items at least 20 years old). Local social networks like Nextdoor have a sales forum, and Facebook also boasts more than 1,000 community yard sale pages with varying levels of activity. There are also local classifieds sites such as Recycler and Oodle. Sellers set the prices and arrange for pickup, usually with no commissions or cuts to the site acting as middleman.