I had an idea that's similar to this. It would be a 1k hat shop where you could buy any of the 1k guild hats. You would have to trade in 2 hats that you bought and pay a large amount of gralats(50k or more) to get a 1k hat. You could choose from any of the 1k hats ever made and when a guild hits 1k the new hat is added to the store. You could also include the 10k mounts for the trade in a mount and 100k. or 50k helicopter for 3 mounts and 1m gralats.

I don't think it hurts the rarity of the items as i don't think many ppl would want to shell out that much or be able to save up that much. You could add to the price and reduce the ability to buy them by putting them in the store on Oasis (vip)island. You could also just have a select few and rotate them monthly to make them more difficult to get.