I've bought and sold through AgTalk classifieds, but it's been a couple years since I did much there.  Recently we've used Facebook marketplace more. Partly because I can mention I'm thinking about something and friends start looking through Marketplace and finding stuff they're just sure I want or need.  There are also many buy / sell groups on Facebook.

In the last few years through Facebook Marketplace we've bought a service truck, a couple semi tractors, a McFarlane Reel Disk, a lowboy semi trailer, a 2 ton flatbed truck, a forklift, a couple tandem axle tank wagons, 330 and 1000 gallon LP tanks, a 35 ft grain platform, and a tandem axle trailer frame.  And that doesn't include my son in law's antique tractor addiction. 

I've sold 3 semi tractors, a one ton dually, a hopper bottom, a ripper, and 3 grain bins. 

Yeah, I guess you could say we've used Marketplace and Facebook groups a lot.

I would suggest working up a bill of sale for anything you buy or sell.  Print it with blanks you fill in by hand if need be.  It should include your as buyer or seller and the same for them.  Names, addresses etc.  Include any warranty or guarantee or a lack of.  If it has a problem disclose it and say buyer acknowledges (whatever it is).  Especially if buying include a statement saying item is free and clear of any liens and seller has the right to sell it.  It should be signed and dated (with time) .  Print 2 copies, one for you and one for them.  It isn't that difficult with a computer.  Keep a copy of it on the computer and just change to fit the situation.

Two reasons:
1) it shows you sold it and they own it.  if they never transfer the title or have an accident on the way home, it's their problem not yours. 
2) it shows they sold it to you.  If there turns out to be a lien against it there will be a problem but you are at least showing of good faith.  I figure if the seller refuses to sign it that a big red flag.  I've never had the problem, but always be prepared.

If they balk just tell them you need it for your tax records and it establishes you jut bought or sold it.  That if you get stopped on the way home you can show the officer you just bought it.  I've never had any problem once I explained why I was doing it that way. I've never said it to a seller, but if they refuse I'm always ready to walk away.

Your biggest problem will be getting paid.  Obviously cash works.  But having a few thousand dollars in cash laying around makes me nervous. I was discussing cashier's checks with a banker a few years ago.  She said she's more comfortable with a personal check than a cashier's check. If you take a personal check with proper ID (take a photo of them and their driver's license) you can pursue them legally.  Cashier's checks are so easily forged with computers these day and so difficult to follow up that she was very very cautious taking them.  I may just be lucky, but I have never had a problem with a personal check on anything I sold on AgTalk.

Talk with your bank about getting paid by ACH.  I sold a semi last year and the buyer did an ACH payment to my checking account.