How do you tell if you got a bum laser assembly?

Bear with me, I have a ton of simple questions... Sorry, just trying to pick some brains of the more knowledgeable!

How do you know if you've gotten bad parts? I tried to fix my son's xbox, which is actually part of his therapy as a special needs child--so it's pretty vital to our household.

Anyway, on a whim, I ordered a second replacement laser and when I went to check out the resistance settings on this one, it was much lower than the one I had gotten before. The other one was set as post pot tweak. The anti-static solder ball was still in place (or was replaced). The other thing that makes me think the last one was used and returned is when I tried to set in the screws on this one, they didn't quite fit and I almost stripped one trying to get it into the plastic. Is that standard to make sure the screws are that tight?

Being a first try at a repair on this, I had no idea what was wrong, I just knew the fix didn't work. The new laser works like a charm. I tried installing the old one several times and reset all the cables a few times as well, checked to make sure I didn't break a solder joint, etc.

Also, I've read where manufacturers are asking for the old laser back--is there some kind of core process for these like in auto parts? How frequently does this happen? How big of a fuss should I make with the previous seller? Is there some kind of core credit available on these?

Thanks for any insight on this one!

Sally