What is the ruling on buying gaming cards from sellers suspected of their cards being stolen or obtained through illicit means?

It is not obligatory to inquire about the source of wealth of someone one deals with, because the default assumption is that what is in people's hands belongs to them, and mere suspicion does not necessitate prohibiting interaction.

As for wealth that exists but was usurped or acquired through impermissible contracts, if a Muslim knows about it, they should avoid it. So, if someone knows that a person stole money, or betrayed a trust, or usurped property, it is not permissible to take that money from them.

The default assumption regarding what is in a Muslim's possession is that it belongs to them if they claim it. If the status of the money is unknown, then the matter should be based on this default assumption.

If a man is known to have unlawful wealth, dealing with him should be avoided out of piety. If most of his wealth is unlawful, there is a difference of opinion among scholars regarding it. As for a discreet Muslim, there is no doubt in dealing with him at all.