The obsession with "genuine" bands is a little strange, but to each their own. If I can get an identical knockoff for pennies on the dollar compared to what Apple charges, I'll take the knockoff every single time. Apple has enough of my money.
I've bought a dozen or so bands off eBay, all knockoffs of existing or previous Apple bands. Quality (and price) have been excellent, no complaints. I cycle among them as it suits me. None have worn or degraded any differently than the couple Apple bands I own.
As someone said up-thread, follow all the same rules you'd use for any eBay purchase: Read the reviews, don't buy from brand new sellers or sellers who advertise pricing that is wildly below the prevailing baseline that others are offering for similar goods.