There is a swignificant difference between a slabbing service and the selling of a slabbed chip.

What is prohibited is me offering to slab your chip for a fee on the floor, but if you had hired me to slab your chip prior to the convention, you would not be prohibited from selling said chip.

It is the services not the actual chips that are prohibited.

If you go back to the post, it states that slabbed chips will not be accepted for the auction.