Exchanging shells is an excellent way to obtain often obscure species that may be the more interesting. You can obtain many items not found (or only rarely) in the commercial shell world, such as Rissoidae and Hydrobiidae. In order to find the names and addresses of fellow-collectors, it is best to join a club or obtain some specialist magazines. Prepare a detailed list of your specimens that you mail to other collectors. You exchange a couple of letters (or fax or e-mail), and when you agree on the species and quantity, you send the shells. The other party does the same.
When it concerns common species, or species without much commercial value, things can go smoothly at a species for species rate. When more valuable shells are involved, it may be necessary to agree on a value, in order to obtain the same value in return.
Quality is often a problem in exchanges, but, just as you look for the best dealer, you should look for the best exchange contact. If time is no problem, the correspondence with the other collectors can give lots of human satisfaction. You can also readily share problems and their solutions. At least in Europe, we have some very interesting and large collections 80 % of which were built up by exchange. In my humble opinion, the combination of exchanging and buying is the fastest method to get a large collection.