For a contract of sale to exist the offer needs to be accepted.
Assuming member 1 offers x-y and it wasn’t an acceptable offer, all be it the first one, it has effectively been rejected (I assume)
The fact that it was first offer is irrelevant.
when member 2 offers x, I assume that offer is accepted, or will be at least as you have received an offer you indicated would be accepted, (the asking price)
If member 1 then offered X after, then it has to be rejected as the offer has already been accepted, or will be. He’s too late.
If you accepted member 1’s offer and then got a better offer from member 2, then sell the item to member 1 at their offer of x-y