I have not heard of that sort of thing lately. They might well take US dollars (or euros or pounds) but I don't think that their back-of-an-envelope exchange rate would be very favourable to the buyer.

As to a discount, it's a bit hard to tell anyway, since almost all prices in the grand bazaar are open to negotiation – it's hard to say what has prompted an apparent "discount".

ETA: In any case, one does need Turkish lira in Turkey – although some tourist shops and so on may take foreign currency, you can't get by with it in ordinary places.