I work in this field.
As far as the Ebay sellers, they are buying merchandise returns from a company that liquidates them for a major retailer. The bigger players are buying products by the truckload (53' tractor trailer container) or multiple truckloads at at time. They are buying the products well below wholesale, sorting, checking, testing, and combining like items to get as much saleable merchandise as they can from each truck. This requires quite a bit of labor and is always a gamble. There is no guarantee that the items will not be broken, used, missing or destroyed. They have no recourse if they are. If it wasn't damaged when the customer returned it, then it will probably get damaged on the long long journey from the return room at the retailer, to the liquidator, and then to the seller. The folks who package this stuff at any step along the way have NO concern for the products as they are not responsible for any damage.
Try and imagine every item that gets brought back to, mailed back to, damaged at, or stolen from (but not the packaging) a big-box store. Now multiply that by how many locations that retailer has. We are talking literally tons and tons of stuff that has to go. The retailer has no use for it, it's in their way.
As far as the one-time Craigslist folks, they were in the right place at the right time and scored on a clearance deal (where the retailer is selling some Weber items far below cost to make room for something else more current or seasonal) and they are buying all they could get, keeping one for themselves and selling the rest for a profit.