Is it wrong...

[M_surinamensis]

Here's a question... When did you buy it?

'cause here's the thing of it in my mind... if you bought it as a neonate and it's now a year and a half old, the seller may not even remember selling it to you, much less the individual snake. Asking for a refund or replacement is kind of out of line if you waited too long before verifying the sex of the animal. What would you ask for as a replacement anyway? A female of the same age? A neonate female?

Even assuming that the newest sexing was correct and that it was (hopefully unintentionally) misrepresented there's a limit to how long a seller should have to stay married to a sale (although if the animal was sold as proven, that's a whole new ballgame). While they have a responsibility to either sex things correctly and verify it a few times prior to selling it as a sexed animal (mistakes can still happen) YOU as a buyer have a responsibility to verify that the purchase you made was the one you intended to make within a reasonable period of time. If the sex mattered at the time of purchase, then you should have verified it at or immediately after the time of purchase for yourself. And this will be a bit harsh but... if you can't verify it yourself (a third party doesn't hurt in matters like this but...) then you have no business breeding animals (or selling them as sexed in a resale situation) and the gender shouldn't matter.