I'll let you in on an open-secret ...
Afternic, Sedo, Ebay, etc are basically advertising venues ... they rarely, if even ever, get involved with enforcing sales agreements...
And legally, it would potentially be very difficult / expensive for them to even do in many instances due to how they conduct business; little to no licensing, lack of diligence, neglecting following various procedures that traditional auction houses / brokers must legally follow in many jurisdictions, etc.
Bottom line is many buy / sell where it's most profitable with little to no regard to agreements of the venues its listed in.
Ron