I think a lot of pirates have a decent stash of doubloons that they can no longer use. Therefore, Reaper's Chests or other treasures that sell for this currency are no longer as sought after due to the low prices of cosmetics that are purchased with these coins.
The Black Market was an interesting mechanic when it was first introduced. It brought time limited items back with a clear approach: "we smuggled these items back for you, but at a high price". Since Rare has mentioned their desire to bring back some of the seasonal items, this could be a nice way to do it.
For example, every month or every season, we get a new collection of items. In the first iteration, we could have 3 items from any of the previous seasons:
- Accomplished Bone Crusher Wheel - 900 doubloons
- Lucky Hand Eyepatch - 300 doubloons
- Snug Scarf Jacket - 450 doubloons
The Black Market could then cycle through some of the previous rewards, allowing players to purchase these items once again. I believe this would still be in line with the Sea of Thieves lore.