It is possible to cut open a polybag and seal it again with tape. If you really
wanted to be neat, you can split them behind the flap on the back and it is almost
invisible. But they are still open and take up more space than putting in a ziplock
bag and most buyers buying a non-sealed set want it for the contents and not
the packet.
Honestly this is why I generally just part them out if I'm going to sell
them.