Selling a whole project with marketplace assets


I have a hard case to solve.
I have a client that wants to buy a whole UE project. He wants to have all the right for the things that are there included. Many things will be made by me but there are stuff bought on marketplace.
I know that he can have access to those assets ONLY for that project BUT I guess he can ask me: “Hey man, I paid for those things, please give me access for those models in case I will lost them.” The assets are on my own account. So how can I send those assets to his own account? I cant give him access to my personal account. If I cant do that (transfer) and he can only use it for that one project, so how can I re-buy the same asset to use in another project? There is no option to buy something more that ones.