I guess my question to you would be what do you mean by "not climbing up in price much"? How much are you asking for the cars on your lot? Links to the ads?
http://forums.pelicanparts.com/porsche-marketplace-discussion/915847-996-tt-prices.html
Most of us would say that they've been climbing quite a bit in recent times. However, many sectors have topped out, at least in the short term. If you are looking to get much more out of them than the prices discussed in that thread, then they will likely sit for a while.