check out a thread posted by torstan yesterday - its all about marketing your mapping skills online
But concidering your question, I'm guessing a BIG question arises... are you selling electronic or printed maps. If you sell printed maps, then you could try selling them through rpg webshops or even amazon. If they are electronic then you can use your own site or again try rpg-shops and forums. Just remember, if anybody sells them for you, they'll want a piece of the profit - and don't be amazed at 50% or more, after all they are doing the marketing for you and actually getting the customers.
So its often the question of getting 50% of something or 100% of nothing