I would price out the AG clothes on the AG website to see what the readymades sell for. I doubt you could get as much for them, even if they are custom made, but maybe 3/4ths of the readymade price would be reasonable? Perhaps with buyer providing the custom fabric?
If they are custom made, I think you would have to insist on payment in advance; you could still have a refund policy that would allow the buyer to make a return (but paying for the shipping, similar to eBay policy).
Also may need to check out what AG-type clothes sell for in places like Target and Walmart. You definitely would want to charge more than that for custom made.