The sweet-spot is a pre-unibody MacBook with a C2D processor. In good condition, you can easily gain about $200-$300 on buying and old one and trading it in. The larger the HDD the better too. Last week I bought a MacBook on ebay for $320 and then traded it in for a $567 GC. Now, I just have to decide what to trade it in for or towards.
One question...if I buy, say, a MacBook Pro from Best Buy brand new and don't open it, will Apple let me return it to an Apple Store for store credit? Apple Stores of course offer some of the other configurable options that the Best Buy stores do not.