I agree...
Mating turtles can cause some damage, but THAT wound would not be what I would consider typical. The fact remains that the seller did not disclose to you that the turtle was injured, and in fact lied to you about its condition.
Sounds like the seller is someone to be avoided, and others would benefit from being told of your experience. By all means, spread the word.
When you think of the transaction in simpler terms, the seller deserves 'negative feedback': merchandise not in condition advertised, not shipped in manner specified, and, seller refused to replace defective merchandise at seller's expense.
Stephanie