My first GSD had only one testicle (Cryptorchid), I found out when my vet checked him at his first visit a few days after I picked him up. The Breeder offered to exchange him, but being new and easily attached to my new baby I decided to keep and castrate him. Lesson learned, I will always check before I buy from now on. He was bought as a pet, so I expected nothing in return from the breeder, I was satisfied with her offer to exchange him. I feel that it is the breeders responsibility to check all male pups in the litter, and if any have retained testicles the information should be shared upon purchase. If the dog is intended as a pet then he should be castrated anyway therefore will cause no problems or surprises in the future, and should not turn off any buyers just looking for a pet. At the time of purchase considering the condition I feel it would be honorable for the breeder to sell the pup for less money to compensate for the difference in price of a cryptorchid neuter as opposed to a normal neuter. The difference is rather minimal $25-$100 depending on the vet. Most breeders will sell "pet quality" pups for less money anyway. This condition is not that uncommon in this breed and the second testicle may very well drop within the first six months. Some may take longer to drop, I've heard of some dropping at nearly 1yr old, however I have read that dogs with testicles that don't drop within the first 6months should not be bred. I personally would not purchase a pup intended for show with one testicle and bank on the chances it will drop, not to say that others wouldn't.
If your dog was sold as "show/breed quality" (on paper ie. warranty, contract or bill of sale stating he was show quality) I feel that you are entitled to some kind of retribution considering your expected intentions with showing/breeding the pup (which should be spelled out if you got a warranty). However contracts and warranties are generally an American way of doing business. If you purchased the dog from overseas its the luck of the draw and you will likely not get or are entitled to anything from the breeder, thats just the way it is.
You live and learn, it still may drop, all you can do is wait and see. However if it doesn't you will at a minimum have to have the retained testicle removed from his abdomen. That is if there is another one, some dogs are only born with one (monorchids). Refer to your vet to find out. If the retained testicle is not removed it will likely become cancerous so it is important to get it out.