Dealer Trade In Service

I am hoping someone can recommend/refer me to a dealer, preferably on Long Island or somewhat nearby, that they know of and/or have worked with personally to help me with the following: I like to buy and sell my cars privately, however, I would still like to be able to take advantage of the sales tax credit generated by "trading". Moreover, I would like to have the ability to hold my "sales tax savings" for a period of time between when I sell my current vehicle and purchase my next vehicle. Is there a dealer that anyone knows of that would perform this service (essentially process the sale of my car to a buyer that I find, hold the "sales tax credit", and process the purchase of the new vehicle that I find so that I can use the "sales tax credit" to offset the purchase price) for some type of modest fee without requiring me to buy/sell entirely through them? How long could I/the dealer "hold" the sales tax credit before I must purchase the new car?