What does first come first serve mean to you?

Part of me just wants to vent, and part of me wants to know if I was actually in the wrong here.

I was selling something at a steep discount and got a lot of interest. I made it clear it was first come, first serve.

Buyer A messaged me in the morning asking if 4pm pickup would work. I said yes, but they never read the message.

At 5pm, Buyer B messaged asking if they could come by in 30 to 35 minutes. We confirmed.

About 10 minutes later, Buyer A suddenly messaged saying he is coming. I told him to not come as someone else was already coming. He said he was literally downstairs and only a few minutes late.

I told him I thought he wasn’t coming and didn’t want to ghost the other buyer but could let him know if it falls through. He got upset because “we had already confirmed and he’s only a few minutes late”and continued to lash out, so I ended up blocking him.

This did get me thinking, technically if he’s already there before the other buyer, one could argue that he did come first. But I don’t feel like we had confirmed. So I just want to see if I was in the wrong here I guess.