Take the small-size version of the Fitbit Charge HR, for example. On Prime Day, it was listed on Amazon for about $143, including a $25 gift card on a future purchase. That base price was similar to the one on Walmart.com, which listed the device as one of its top-selling items Wednesday, when it hosted a rival sales event. On Thursday, the same Fitbit was being sold on Amazon for about $117.
Other items were going for the same price Thursday as they were Wednesday. A Rusk Speed Freak ceramic hair dryer that was listed at 61 percent off Wednesday offered the same savings Thursday. So did an eau de parfum spray by Chloé.
Read MoreCustomers unhappy with Amazon Prime Day
"Some deals will continue today on the few items that did not sell out," Amazon spokeswoman Julie Law said. "In addition to offering our customers great value through Prime Day deals, we are working to always offer our customers low prices on all products, every day."
These findings come as consumers expressed disappointment in Prime Day's deals, which were touted as more extensive than the retailer's Black Friday event.
Analysis from Clavis Insight found that while Prime Day offered "far more deals in a broader range of categories than usual," the discount levels were similar. Kurt Salmon director Steve Osburn noted that most items were an additional 20 to 30 percent off of Amazon's regular price, which is in line with the company's typical "deal of the day" items.
"The product mix was somewhat esoteric, deals were too mundane and the website was difficult to navigate," FBIC analyst Deborah Weinswig wrote after the event.
Read MorePrime Day: It's not all about Wal-Mart
That's not to say that all the deals were duds. The Kindle Fire HD 7, for example, was marked down to $79 on Prime Day, and cost $139 Thursday.
Despite shoppers' disappointment, Amazon said it sold more units on Prime Day than its biggest Black Friday ever, and had more new members try Prime worldwide than any day in its history. The retailer said customers ordered 34.4 million items, buying 398 items per second.
"The bottom line: In the first time trying this event, Amazon has created a summer shopping holiday as big as Black Friday," said ChannelAdvisor Executive Chairman Scot Wingo. "Amazon has exceeded our expectations for Prime Day."
Amazon Prime Vice President Greg Greeley said the retailer will "definitely be doing this again."