These days even cash can be fairly easily counterfeited, and those dye pens don't mean ****. THERE IS NO WAY that I'd accept that kind of cash money without taking it to a bank and having it inspected
I sold a four wheeler once for $4000 cash but ended up getting $3700 for it because the bank confiscated three of the bills saying that they were fake. I got into a big pissing match with the buyer because "those are the bills that his bank gave him". My only final dig was to tell him that I gave his name and information to my bank so that they could give it to the treasury department. Of course, I never heard anything back.
Get a certified bank check and go deposit it in your bank. Your bank will call and verify that the funds are there, and once deposited, you can release the title. CASH is not king on big ticket items.