Not universally. The best option for offloading an individual item will probably change depending on circumstances; it's up to the seller to decide if the tradeoff of doing research vs finding the most convenient option is worthwhile.
Here's an example: Uncharted 2 trades in for $0.25 at both
GameStop and
Best Buy. At that price, you might be insulted and think that you'd be better off selling on Amazon.
As previously mentioned, Amazon charges $2.64 in flat fees as well as an additional 15% on the selling fee, which is independent of the $3.99 shipping they require for sellers without a storefront, to the best of my recollection. USPS First Class shipping for packages under 3 ounces is $2.45, so the formula comes out to:
profit = item price*0.85 (variable fees) +3.99 (shipping credit) -2.64 (fixed fees) -2.45 (shipping cost)
If you solve for the break-even price, you'd find that you need to price your item at $5.29 just to avoid losing money on the transaction. Of course, at that price, you'd have
about a page of sellers who beat you on price. To make 25 cents of profit on the sale, to match trading it in, you'll have to charge $5.58.
This isn't even an example that's particularly favorable to trading in. Let me know what you'd sell on Amazon and I'll tell you what you could be doing with it.