I have a 3GS which is pretty flawless and a 4S with a shattered back... Do you think they'll care about the 4S's back?
Also, I always wonder about these products becoming vintage. If I hold onto them for 3-4 decades, will they be worth a lot more than (present day) $100? What about my iMac and MBA? They're so old that I don't think their values will drop much more, but if I keep them in working condition over that time, will their value go up substantially?
It all depends on the market at the time you choose to sell. With the legion of Apple fans your chances are 60/40 at getting a premium. I say that because chances are also pretty good a fair number of Apple fans have the exact same phone in the exact same condition. The collectibles market is horribly fickle. Your 3GS could be the next Beanie Baby
On topic: Not a bad deal from BB. Removes a lot of barriers by not restricting the trade-ins to iPhones. A customer can potentially gain a less expensive new iPhone all for the cost of a "drawer phone" with little to no value. Well played Best Buy. Well played.