The competition aspect has been one of the more frustrating/interesting elements. After being threatened with tougher gating restrictions, it seems to be easier to sell on Amazon than ever. When Walmart had that $25 sale on Queen Whatevra's palace, within a week there were close to 150 listings. (Which, for those uninitiated, is a lot. A lot a lot.)
Even the Nebulon B, it was funny to watch as everyone was reporting getting their shipments yesterday, the sellers went from 1 to 9 in a couple of hours. (Down to 7 currently).
Mandalorian Battle Pack (still in production) has 133 sellers right now. These are not the sort of numbers we have seen in years past. There are a LOT more sellers much more willing to quick flip for smaller profits and engage in undercutting wars.
And LEGO themselves, sometimes I wonder with stuff like Treehouse and Diner, they are well aware resellers are buying these in large quantity - and to me, feels like they are basically printing money by keep popping them out and seeing them snapped up. Treehouse has 77 listing, Diner 50. And this isn't counting folks hoarding them.
Obviously these metrics aren't the be all end all, but I think they are somewhat telling of things just being different - Obviously, more competition and more supply makes it harder for resellers.
I don't really have any advice that hasn't already been given, just kind of echoing how things are definitely different than they were in years past, which is to be expected of course as folks see the potential for profit and it gets more exposure. I personally kind of believed Amazon and LEGO when they said they wanted to make it tougher for resellers, but their actual policies haven't really borne that out.
Other retailers, like Target, have expanded their LEGO selection but definitely changed how they clearance. I've seen several Wal-Mart stores (mostly smaller ones, but still) actually reduce their LEGO shelf space.
As always, it's an interesting time to be reselling.