Finding a similar bike on eBay is going to be more accurate than any price guide.
A bike is worth what someone somewhere is willing to pay for. The more eyes that see the bike, the higher the potential price. So a bike sold for "local buyers only" is unlikely to sell for the same price if the same bike were sold "nation-wide."
Also, the price you'll get will vary wildly based on what other options buyers have. If there are lots of buyers in your town, and you're the only seller, you'll do well. If on the other hand there are many sellers of bikes that are similar to yours (i.e. the same component range) and few buyers you'll get much less money.
Use eBay to get an estimate, and then be prepared to sell if for that price +/- 25% of the eBay price - and probably less if you sell it locally only (unless someone locally really wants "that" bike).
If a price guide says your Trek is worth $1000 but someone on Craigslist is selling a similarly equipped Giant for $500. Your bike will not sell.
