Hi Reba!
Haslina described it very well. Wanted to add that some buyers don't accept personal checks, only money orders or an Internet method called Paypal. It all depends on the buyer. And also, fyi, I've purchased a number of tapes on the Firm Swap (though I've never traded for tapes), and all my transactions have gone smoothly and the tapes I've received have all been in good shape.
Good luck!