I doubt any seller could do much. This member even went to Poland and asked for his money at a branch. They didn't give him his money: https://bitcointalk.org/index.php?topic=746065.msg8905377#msg8905377

Look in that thread, there is a list of emails on two or three posts. At least a member got his money back, by spamming those emails with complains, including regulators.

Or join with other members and hire a polish lawyer to threat Mbank with a class action. A simple threat will do, it won't be necessary to go to court, so cheaper fees of the lawyer.