I don't know any way of checking the keys that were used to create a multi-sig address and how many private keys are needed to release the funds without the redeemscript. (If they exist, please tell me).
So in other words, if markets do not offer those, they could be scamming all of us. Since nobody uses multi-sig (despite claiming it was the future when Evo exit scammed), I don't know if this information is available or not.
Does anybody know?
I cannot speak for other markets but Oxygen does. Both, the buyer and seller get the redeemscript together with instructions.