Hi there.
I am one of two wordpress developers. We are working on themes bought on Theme Forest.
We have an account with some themes already bought, but not so long ago we found out that one of top wordpress themes meets or almost meets all our and our clients requirements.
For now I have already bought this theme 3 times, and I am developing 4 another websites on this particular theme. It is 7 times in last two months.
I am starting to get confused with my purchases list. By the end of the year I will have probably 10 or 15 of this theme bought and now, having 3 of them I am confused which one was bought to whom, who owns which key and so on. I can see a disaster coming. Later I will be confused probably whether I bought theme for my customer already or not 
Do you have any type of method how to manage this? This is completely new problem for me.