As a native speaker, I would use either. They are both correct. In my opinion there is a very subtle difference in meaning, and I would choose one over the other if I required that difference in emphasis to be clear to the recipient.
Could you sell your old computer to me, seeing (that) you have no use for it.
Implies I understand the owner of the computer has a choice of whether to sell or not and I'm asking if he might choose to favour me as his chosen buyer, rather than keep it or sell it elsewhere.
Can you sell your old computer to me, seeing (that) you have no use for it.
Implies that there might be constraints on selling the computer, and I'm asking if it is permissible. Perhaps the computer is still needed, or that the other party may not actually be the owner.
However, even many native speakers may not be cognisant of the nuances in meaning.