I find leopard geckos, African fat tails and gargoyle geckos (if you're already breeding cresteds) are easy to breed. I have found that just about anything is easy to sell if you:
--don't produce huge numbers
--sell at reasonable prices, though not prices that seriously undercut everyone else
--provide healthy geckos, old enough to be well established
--provide excellent customer service which includes not pushing people to buy and being a resource for questions after they've bought.
I'm in my 8th season and have been able to "sell out" every year so far.
Aliza