Whilst I have not done this myself, I have read several times on the official forums that other players have. If you have the items the ships are seeking, they are magically taken from you and you are automatically paid for them.
I can only assume that you get paid a considerable amount more than you would get selling them at any station or you could end up quite upset if you lost out on potential profit.
This wouldn't stop situations where you had bought the cargo for a "Please bring me X tons of Y" type mission from a bulletin board and the cargo was taken from you.
Edit: Confirmation of the "fly close" mechanic.
This thread also clarifies what is a "luxury item", in this case players are trading Progenitor Cells with these mobile markets.