Corn snakes and leopard geckos are in my opinion the 2 easiest reptiles to breed.. rarely have problems, usually get pregnant with 1 mating, usually good eaters so easier to have good weighty females and neither require brumation (although some people will say different..) Short incubation time too.
On top of that, the added bonus is you'll never be stuck with babies you can't sell because they are hugely popular and if you can't sell them privately, pet shops will buy them in bulk.
Beardies on the other hand take up more space and I have found them a little harder to breed - they eat a lot more to get into breeding condition, and generally do better if cooled for the winter. The eggs also take longer to incubate but you do tend to get quite a lot of them! They like the above 2 are easy to sell.
Corn snakes can be sold at 1-2 weeks if they've shed and fed, leopard geckos usually take 2-3 weeks before they're feeding and growing fast and beardies I would say take at least 5-6 weeks (and they need UVB for this time) before ready to be sold.
Not many people seem to breed water dragons, I'm not sure if it's because it's hard to find the adults, hard to give big enough cages, or just no demand for them - because they are imported in huge number.
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