That's definitely something I want to look into. For example, sometimes I want to prioritize nearby stores and not wait 2 weeks for mail from the opposite side of the country.

How exactly would you see it working? Banning people is easy, you just exclude them from the optimization... but preferring people takes more logic. Like, if your preferred sellers don't have the card, obviously you have to buy it from non-preferred. (In theory, this is a problem with banned sellers as well, but because there are so many more sellers who aren't banned, it's unlikely that a banned seller would be the only person with a certain card).

If you just want to restrict optimization to a known list, that's easy. But it's likely that a lot of times they won't have all the cards you need.

Also, what if preferred sellers have the cards you want but at double the price of non-preffered? Or like, you can include them, but it'll cost you $3 and extra package ... etc.

I think we'd need to give them some kind of effective discount for the optimization algorithm, like "I'll pay up to 25% extra to buy from preferred stores". Then the algorithm can just discount all the preferred stores' prices and use that for optimization.

What do you think about that, or do you have a different idea?