I think the nice answer would be to offer the seller an animal or two.

But really, the seller sort fo HAD to be aware that she was possibly gravid, right? Either s/he sold her too quickly after s/he bought her or s/he was the one breeding her. I don't think there's any moral obligation to give the seller anything unless you just feel like being very very nice.

And really, the seller shouldn't be selling an animal that could be gravid without warning the buyer. Not everyone is ready for unexpected eggs!!