Funny you would ask.
I sell engines, garden tractors, farm tractors, and parts.
If you want tales of "pay and no show" I can delve deep.
I did have an old Wisconsin engine that took me two years to sell. One man said "I can't live without it!" and sent 50% of my asking price. He said he would come see me. 6 months later, numerous phone calls and emails later, nobody ever showed so up it went for sale again. Another hit. I was impressed. This restorer needed it for an old garden tractor. I said sold and asked for a 50% deposit. He paid and I waited for him to arrange shipping since he claimed he had the contacts to make shipping quick and dirty. The phone never rang. A year later, while cleaning up shelves, I saw that engine and remembered. I called the poor guy up and he did remember me but he said he didn't need the engine after all. "Keep the deposit" said he. I once again put it up for sale, and nobody but man #1 who had originally paid 50% down calls me up and says, once again, that it was the dream of a lifetime to put the engine in his machinery. I told him I remembered him from way long back, and that all he owed was 50% of the price and the engine was his. He sounded shocked, and said I was thinking of someone else. I said, "No, you are the one who paid, I have a photocopy of the check right here in my files." He said that it was someone else, he was sending 100% payment, and he wanted the engine shipped. I finally calmed him down enough to send him a scan of the check. He finally conceded that he had indeed forgotten me nearly 1 1/2 years ago.
That's tame. I have a deposit down on three machines, two farm tractors and one garden tractor, with neither hide nor hair of the prospective owners in sight or accessible via phone.
My rule has always been to give plenty, plenty, plenty of time. If you do decide to resell, make sure you have a paper trail of what you did, how many times you called, letters you sent, emails you sent. Get it notarized. Then do the poor man's defense of sealing it up in a nice manila envelope, take it to the post office, have them date stamp the stamp and the seal of the envelope, and let them mail it back to your house. Save it just like that, sealed and dated, so if the man with the deposit comes to get you for robbing him of the money, you have something to go back on.
That's what I was told by a lawyer, anyways. Take it for what is worth. It ain't my two cents, its a lawyer's five bucks.
I've been there, done this, felt this, so many times it isn't funny.