Experience with Adio Equipment Trade-Up Promotions

Anyone have experience with vendor "trade-up" promotions where they buy your used equipment and apply your trade-in as store credit towards a more expensive piece of equipment they are selling?  Are you much better off selling your used equipment elsewhere (e.g. Audiogon) even if you intend to buy equipment from the vendor (e.g. MusicDirect)?

I'm curious on your experiences.  Thanks in advance!