This article is from the Macintosh for sale FAQ, by Elliotte Rusty Harold elharo@shock.njit.edu with numerous contributions by others.
You can't, which is the single biggest reason I recommend
against buying and selling on Usenet. The most effective
precaution you can take is to insist on check-on-delivery. That
way you give the seller nothing until something is delivered, and
you can stop payment on the check if what is delivered is not what
was advertised. This may cost a few dollars more, but the added
security is more than worth it. NEVER pay in advance for used
goods from an unknown shipper.
You may also want to insist on a work address and phone number
for the seller. This is especially effective if the forsale post
originates from a company account. The threat of being harassed at
work is more effective than the threat of legal action against some
unscrupulous individuals who realize you probably won't sue but may
complain to their boss. Even when dealing with completely honest
individuals it's still much easier to haggle over terms and work
out shipping arrangements via phone than through protracted
email exchanges.
 
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