lets see...
buying a boss amp is a huge loss no matter how you look at it
buying an amp off ebay is only a loss if something happens to it
How many of you have had to have a car audio component replaced/repaired while it was still under warantee? I have bought quite a few things at stores and online and only once did I have to have something replaced, and that was within 7 days of when I purchased it. Somehow I fried one of the V.C.s on my Eclipse sub. Other than that, I have had very good luck.
Anyway, one thing you need to pay attention to with low-end brands is power rating. Usually they advertise the peak power handling and even the RMS power handing that they report is inacurate. High end companies usually underrate their amps and come with a birth certificate as to how much power they really put out. They also have better powersupplies which guarantee you get that power all the time. A better amp will also work more efficiently at low impediances, usually doubling their output between 4 and 2 ohms. So taking all that into account, two amps you thought were equivalent may end up like so:
BOSS "100x2 Amplifier" 50x2 RMS in small writing = 45x2 RMS, ~140x1 RMS
a/d/s "100x2 Amplifier" means 100x2 = 100x2 RMS, 400x1 RMS
So there is no comparison between a 400 watt amp and a 140 watt amp...regardless of THD. Then when you factor in build quality, over all sound quality and everything else...the a/d/s amp is worth the extra bucks, even if its used and has a few scratches on it.