I disagree. I bought a 2009 Macbook Pro 13 inch and opted for the more expensive model. When I went to sell it, I didn't get any more money for my machine then those people selling the cheaper model. The only difference was the cheaper one had 2.1ghz instead of 2.53ghz, and 2 gigs of RAM instead of 4, but that didn't seem to be a consideration for most people.
I think people have a perceived value of a Macbook pro on Ebay and Craigslist and the majority of buyers have a certain price point in mind and could care less about specs... It seems to be around $800 for the 13 inch model (previous gens), $1000 for the 15 inch model, and $1200 for the 17 inch. These prices I'm quoting are for 2009 and 2010 models, obviously if you're selling the newest model 2011, you can expect to probably get 90% of retail for your machine...
Feel free to disprove me with some actual data you're seeing on your local Craigslist, or ebay auctions.