Every time I've bought and sold something on FB Marketplace, I've never left or taken a deposit. It's always been 100% cash paid on collection .... that way, there's no risk of being scammed etc. The only risk is if they don't turn up, then you have the slight inconvenience of having to relist it. Most times though, there will be several people interested and I'll simply let the second person in the queue know that someone is meant to be collecting it and if they don't turn up, you'll let them know :)