In my case, I often sell gear that I have gotten as an REI employee on super special discounts direct from manufacturers. There are certain set-in-stone rules for such deals:

We have to use the gear at least once and be willing to write a review if asked (though I've only written a review a couple of times in 7 years).

The gear must be for ME, not someone else.

I must keep the item for at least a year.

And I can not sell any thing for more than I paid.

This is where a lot of gear I offer up for sale at big discounts comes from. If I get a pack that I don't like so much, or if I get a newer piece of gear that I like even better, I'll often sell off gear at ther prodeal price minus a bit more for reasonable use. Buyers are usually getting a very lightly used piece of gear for very cheap.

If someone is selling LOTS of gear from the same company, they may work for a retailer and are selling gear they purchased, then didn't care for. Or they may be abusing the prodeal program. REI actually monitors REI gear sold on EBAY. If they see a pattern from specific sellers, they'll investigate to see if this is an REI employee abusing the program. If so, the seller can be fired