The real risk in mail order is when dealing with an unknown individual. We are not being critical here - there is nothing wrong with individual collectors selling their own books via the trade papers or over the Internet. The individuals collected money for a few weeks from an ad, then left town. One was caught by Postal Inspectors and prosecuted, but I never received my money back on either transaction. Ordering is easy.
Most collectors call and check to see if the items they are interested in are still available.
Some collectors simply write a letter listing the items they want and mail it along with payment. The majority of collectors that do not reserve items list alternates, in the event their first choices are already sold.