I don't agree with that, osclass get 30% commision to use the market, the support and updates must be given by the creator (he get 70% of the price). He should give you the support and updates because you paid to him for the plugin. As i said, any problem between osclass and mb-themes doesn't concern to you but only between them.
If is not to long that you buy the plugin you can open a dispute with paypal and try to get your money back, then, if you want, you can buy it again from mb market.