Most of the time, ebay is great. Deal with people with solid feedback, and make sure you e-mail them. Even if the product comes in as broken and unworkable, just tell the seller and many time he'll try to figure out how things can work out...
OTOH...getting an ART Tube...the consistency on those things isn't too good. I have on
ART Tube MP OPL And it sounds pretty darn good...which is much more than can be said for my friend's first Tube MP Studio. He was able to return it for another one that sounded fine, however. If you really want an ART Tube MP, I'd go to the store and make sure I can go back to return it if it sounds like crap.
If you're looking for under 100 dollars, check out
the MAudio Audiobuddy. it sound pretty good, and is a two channel preamp, even. The cheapest per channel solution out there, outside of a Behringer mixer. The only thing that people talk about with the Audiobuddy is the usefulness of the Phantom power in powering more extreme beasts, like the CAD mics. Since you're on an SM57...no worries about that!
Actually,
my Behringer UB1002 doesn't sound bad! Just picked it up for routing my synths around, and preamps, which I originally thought would be EHHH at best, are quite clean.