You can rotate the untradeables throughout the weeks. But, only problem is that I'm not sure how sold this server is on custom rares.
Doesn't necessarily need to be customs. Use items that are already available in the game.
Scythe if not already obtainable, Skeleton suit, Chicken suit, etc. There's tons of options for non-tradable cosmetics that could serve a purpose as a money sink.
- Add a tax to converting GP from gold to platinum.
I would shy away from the Gambler NPC idea. I tried that once and I wish I had never even dreamed up the idea. Brought way more gold in than it removed.
- Could make a cosmetic shop, Scythe or other sought-after untradables. Could make the price something like 50m or so. Enough to be attractive enough whilst still maintaining the idea of a money sink.