If you take the opposite line, that Facebook is responsible for checking that the product is sold legally, then the conclusion is that Facebook would have to confirm legal ownership of every product before allowing a sale.

This also seems wrong.

The right solution is in the middle: Facebook should spend some effort validating the legality of the sale. The required standard of effort should depend on how much money Facebook is making from the ads, and on an estimate of the proportion of products sold illegally, amongst other things. The former to ensure that a very small similar website is not held to the same standard, the latter to ensure that if 99% of the products are sold illegally then Facebook is responsible.

Without knowing the total volume of these land ads and the proportion of illegal ones, it is hard to tell if there is anything to fix here.