I have a problem, that might be similar.
All the apps that need purchase, but that I have downloaded on a 3rd party market, like getjar, are marked as purchased in the play store, and it always ask for an update, like Beautiful Widgets. So, if I want to purchase it, I can't, because it is considered as purchased already.
Does anybody have the same issue ?
I hope my message is clear enough so you could understand ^^