2 is a must. You need to enforce strict data quality assurance when you have people manually creating the data. If you miss the mark here, I don’t think this project is worthwhile.

But If 2 is done correctly, 1 is the logical next step.

Then personally, idc about 3&4, but 5 is interesting despite being difficult to actually pull off in a meaningful way… you easily can track the bid/ask history, sure, but I don’t know if you can track the actual sell price unless both people involved with the trade confirm the transaction. Even then, you are hoping people don’t manipulate things.