I bought my first two (and thus far only) Magic Bands off ebay. One experience was bad, the other was good. It's all just a matter of being careful.
I admittedly wasn't careful enough on my first purchase...
I wanted a Tinker Bell band, and I wanted it before my trip started, because I wanted to be ready to hit the ground running once I got to Disney... at the time, Tink bands weren't available online (they are now) so ebay was pretty much my only option if I didn't want to wait 'til I arrived at the park.
The first one I bought was sealed, which I (in my then uneducated belief) thought meant it would be fine for use. When I tried to link it to my account it was a no-go. I really have no idea why to this day, it may be as simple as the seller did not have it set to "link it later" mode when they originally acquired it. I called Disney and they claimed they could do nothing, since I was honest w/ them about where I bought it. Seller claims she called Disney and was told they could fix the problem and link it in the park if I brought it in with me. The seller didn't seem shady. Good feedback. Good communication. BUT, I was stuck with a (now open) useless magic band, and it was her claim that she couldn't return it because then she would be stuck with a now open and unsellable magic band, so after some hemming and hawing, we inevitably split the difference and she refunded half, but I kept the band. I keep telling myself to bring it to the park and try to get the problem fixed, but thus far never have.
THAT said, still wanting a Tink band before my trip, I bought another one, from another seller, and made certain that the second time I found a listing that SPECIFICALLY said the band was guaranteed to be linkable, and set to "link it later" mode. So if you by one on ebay, be CERTAIN you buy one that STATES THAT. That's my advice.
I've been using the second band ever since. I have the first in an envelope somewhere.
I'm going to take it with me on my trip in Sept. and see if the cast-members in the park might be more helpful than the person I spoke w/ on the phone. I still have the original packaging, but obviously not a receipt. Best case scenario I'd like to exchange it for a different band of the same price point (since I now have a replacement Tink band already), but I feel that's probably overly optimistic to hope for. But sometimes if you get the right manager, on the right day, retailers will be friendly and do things like that. And Disney is a retailer that often aims to please its guests. Acceptable scenario: I at least get the second (or more correctly, first) Tink band linked to my account, and I now have a spare. Worst Scenario: They do nothing, and I still have a useless bracelet I paid too much for (but at least I'm no worse off, haha).