While inventories are still very low, they have come up about 6 percent from January to August, and that may ease price gains as well.
"With survey evidence pointing to a significant increase in the share of homeowners who view this as a good time to be marketing and selling a home, housing inventory will continue to rise," noted Paul Diggle at Capital Economics. He is predicting just an 8 percent annual gain in home prices for all of 2013.
The wild card continues to be investors, who made up anywhere from 33 to 45 percent of homebuyers in September, depending on varying surveys. With prices rising and fewer distressed homes on the market, some investors are pulling back on purchases. The question remains, what will they do with the thousands of properties they already own?
"All these institutional investors are going to behave differently than the homeowner of the past," noted Shiller. "That's another reason to suspect that there could be a rapid turn down in the housing market, because these investors are not going to sit tight when home prices start falling. If they think they're going to continue falling, they'll get out."
(Read more: Home affordability sinks as housing slows)