Welcome to forum Jesse.
IMHO, this seems like a reasonably fair deal on both ends. I think that is a decent price for both the K20D and the K5, particular in view of the fact that you need do nothing more than a few mouse clicks. Yes, you might be able to do a few dollars better on either end, but with much more effort invested. I personally have no experience with the Amazon trade-in, but overall Amazon does seem to do what they say they are going to do. Another option for selling your K20D would be to investigate what KEH (
http://www.keh.com/ ) offers for it, if nothing else it will provide a benchmark number to judge the Amazon deal by. On the other end, buying from Adorama is a pretty sure thing, I have have had many transactions with them without any issues whatsoever.
Regardless, I'm sure the K5 will be a big help in your efforts, good luck.
Matt