Ubercart itself does not seem to have any problem with deleting a product after someone has ordered it. It stores all the information it needs about the product so that reports about past sales won't be affected by changes to any products.

However, marketplace looks up the node in an order to see who the owner/seller is. That means that if a product is deleted after someone has ordered it, that product will no longer show up in reports. In the short term, that means it's possible for a seller to not get paid for a product they sold. In the longer term, it makes balancing the books at the end of the year more difficult.

The obvious solution is "don't delete products...ever," but that's not exactly ideal, especially when it seems like it would be easy to just connect the seller's UID with an order line item at the time a user checks out.