I'd definetely start with a proofen pair that already had fry. That way you don't have to guess in a store which ones are the males and females. And I would also have the tank completely cycled before attempting to breed, unless of course the fish decide not to wait that long....
Also a very good idea to stick some extra sponge filter into that tank as well, so whenever you are ready for the job of a goldfish father, the fry have a cycled sponge filter for them. I personally like the hydro sponges, those things come in all sizes, are cheap, and have a big surface area for the bio bugs to live in...

They are being sold at most stores, online and otherwise...
Hydro sponge filterAs for where to sell them too once they are old enough, there are quite a few possibilities. I personally see the pet store as last resort, since you don't have any control over who gets your fish. They might end up with somebody who doesn't have a clue on how to take care of them. You can look in your circle of friends and see if anybody there "needs" a goldfish, or find a local koi/goldfish/water garden club or aquarium society for any fry - those people know all about fish keeping...