i've never "e-bayed" IT, but if you feel comfortable with it, go right on ahead. Just make sure it has a warranty... Some people will tell you it has a warranty, and then something breaks down a year later and you come to find out it has none... I'm not saying this is the case, but this dude's sales are final.
And to answer your other question, yes, you can do just 5.1 and not the 7.1. My receiver is 7.1 capable but I only run it in 5.1. Anyways, enjoy!