(5e)

I'm currently running a campaign where there aren't any magic items available aside from random treasure, the party's only spell caster is a paladin, and they have a ton of money (including gemstones) that they haven't had any chance to spend yet. I know they also love to goof around and I honestly want them to feel rewarded for playing and give money and the gemstones meaning.

So, knowing that, I want to add a store or something where they can access alternative, weird, funny and cool homebrew items they wouldn't be normally be able to access anywhere else. They would still get the normal treasure from the DMG but I'd give them an option to skip that and get that one sword that lets the player do an extra 1d4 damage if they manage to make the DM facepalm with a one-liner or some really overpriced Flaming Poisoning Raging Sword of Doom.

Does this sound like fun? And if so, how do I go about doing this?

So far I've thought about them rescuing an unaligned crazy witch that travels between planes who will later visit the party to offer them weird items to buy. But I'd also like to limit this, for example:

• Every in-game month I randomly add 1d20 items to the list but also remove 1d20 items. (They can come back later)
• The witch only accepts gemstones as payment.
• Trading is accepted (DMs discretion)
• Players can submit their own homebrew items they want to buy and will be added to the list, but with the risk of getting lost each month.
• As long as there aren't enemies nearby the party can call upon the witch with X item.

Is this too much? Would you do this differently? Any suggestions for strange/funny/cool items to add?