If it's working and in order, yes, $50 is at least a fair deal, and if there's no shipping in order, than the deal is in your favor. The 5700 is a great amp- I own two of them. My questioning in the beginning was basically wondering if I'd gain a ton by going to a 6100. I think now after thinking it out, feeling it out, you'd gain minimal (a touch more power and a dual power source to help eliminate crosstalk), but in all reality, it's not that much of a difference. The 5700 is a great amp, and at a conservative 40wpc, sounds much more powerfull than it is listed to be.
I wouldn't think twice at that price. really. It's a fair deal at worst case.
good luck!