Welcome to the forum. The ALA has a lot to offer in terms of knowledgeable people.
When it comes to guppies I am not one of those knowledgeable people but I do have a favorite. I love Endlers livebearers. It’s actually the only guppy that I have kept. They seem to be smaller than many domestic strains. They also seem to sell really well at club events like auctions and swap meets.
Which brings me to the next part of your question. I am not sure were you are from but try to locate a local club. (look in the back of an aquarium magazine as they have lists by state) You will probably find much better fish for breeding from local club members, and of course ALA members, than you will at most pet stores. The difference between the farmed fish and the locally breed fish is very noticeable. I know that local stores in my area like when they can get high quality fish from local breeders and that includes guppies as they are always good sellers.
As for what you can sell back to stores you will have to ask them. Try to find some mom and pop stores and ask them how much credit for each fish you can expect. It usually is not very much unless you have very good fish and they can sell them for a premium which sounds like what you would like to do.
When it comes to livebearers I mostly keep and breed Goodeids and apparently Limias as I always seem to have 2-3 species swimming around my fish room. A good resource for goodeids is www.goodeiden.com
, but that doesn't really help you with guppies.