As far as I'm concerned, the seller can sell to whomever they choose. Until they accept payment from a buyer, its up to them to choose the buyer. There is no "line" or "I messaged first". People offer to buy things all the time, then stall with payment or ask a zillion questions. Some sellers list a part somewhere else too. Nothing wrong with that, but if the seller wants to sell to someone else, its totally up to them.
I only care that people who actually pay, get what they pay for, and get it in a reasonable period of time.
