Plan for the future, but be ready for your next game. You can only play one game at a time. Some older players are cheap, good and is just want you need right now.
I've never sold a 7 star player myself, I can't think of a situation where I would. Spent so much time, training and tokens getting a 7*. I too wonder what happens to a sold 7/8 Star player, I think they are put on the transfer list at a higher level.
Yes Negotiation will appear in level 4, you can just go back to day one matches, click the sold player, bid button and check the club that bought him, I'm talking PC version of course. Tough, no can do on a phone.