I'd second what Mitch2 says. And event though a fair proportion of participants will treat our specialized markets as communities, there will still be traders that will prioritize any sale over respectful arrangements.
At the other other extreme is Facebook Marketplace - a total Wild-West of a trading place. To lock in a deal - you have to show up with the money, on-time; offer to pay a reasonable deposit with Venmo ahead of pick-up; or get a firm commitment from the seller that they will hold, like "Will you hold for 3 days until I can get there in person?". This latter approach can be a bit hit or miss - I have lost out a couple of times because of it.
So, the two things you could do differently next time are 1) pay a deposit with PayPal/Venmo, 2) get a commitment from the seller that they will hold for 3 days, even if another offer comes in.
Best of luck in the future ....

