Because rare species need to be bred more, they dont need to be sold in singles as pets as that doesnt help the preservation of the species. Now a lot of breeders will sell a single if you talk to them and you already have half of the pair but otherwise it's better to sell in pairs. And as for breeding, reptiles can inbreed for a little while before it affects anything. Yes it's better to mix bloodlines to make new blood and decrease inbreeding but sometimes that's not always possible with some species. The ideal situation would be you buy the pair and switch one half with someone else to better each others blood, but it's not the end of the world if you have to breed siblings together.
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