I feel this needs the following:

If buying, using online payment/ buying with postage required. Use paypal, and make sure you send the money through for goods and services, cause if you send it as a gift, and you get ripped off, you can't get your money back. goods and services lets you make a case against them if it's not as they described or something.

And for postage; ALWAYS get tracking numbers. If you're selling especially. ALWAYS ALWAYS ALWAYS use tracking numbers. I've had one person scam me out of an item by saying it never arrived. And without tracking, you can't prove it made it to their end.
Also helps for if it gets lost in transit, you can tell/ they can find it easily

Excuse the necro