eBay being the biggest eMarket, has 3-7 trek bikes listed at any given point of time. But shipping may cause damage. Similarly oldroads.com provides a space to put up a ‘wanted to buy’ post. Similar sites which allow a wanted post are Classic Rendezvous and the famous craigslist.com.
Subscribing to a mailing list will expand your options and choices to choose from. iBOB and bikelist.org provide mailers related to trek bikes. Bike swap meets is a good place to find bikes. You can find out about such meets on the Internet or also some local bike shops. Garage Sales and Rummage Sales, Goodwill, St. Vincent de Paul Stores, The Salvation Army, Value Village, Savers, Thrift Town, and others are good places to look for used bikes.
All the above resources are also useful for selling adding to them, Sheldon Brown's Guide to Selling your Used Bike http://sheldonbrown.com/selling. Finally a closing tip for selling is ‘Road bikes typically sell for more money and more quickly in the Spring and early Summer.’
You can find tons of resources related to vintage trek bike buying and selling here - http://www.vintage-trek.com/buy_sell.htm
