Well, it's true "first come first served" but not if the seller agrees to meet the buyer
on a certain day - this becomes a verbal contract and thus an obligation to the buyer to hold the stuff.
What to do in cases like that, you can declare "sorry, no hold policy" so no promises
the stuff will be held until then. then ask to call before coming to see if there is still worth coming over.
A 'No' uttered from the deepest conviction is better than a 'Yes' merely uttered to please, or worse, to avoid trouble.