As I did gain a lot of experience selling my players on negotiations, I thought I share my thoughts here in this thread.

So far I sold around 50-60 players through negotiations since the new transfer market. How? Because I always swap these 55-59% 3 star players and 75%-79% upon to the age of 20 (older only if 3 positions or SA) for quick profit.
It's a really good business for me and I provide other players with good young players, which otherwise would just be deleted through the market.
I always have 4-6 of such players in my squad, which also boosts my training level as well.

I try to don't waste to much time waiting for a good offer. If I get 6 or 7 tokens for a really good 3* player, maybe 18 with SA, its fine because generally I only spent 1T on him and by selling him, I make space to do the procedure again with another player.
So my advice is to not be too greedy. If you sell 4 players for 4T profit each in 2 days its better than waiting a long time for a 16T offer on a phenomenal player.

This season I sold so far:

ST AML AMR 19 years old without SA for 10 tokens
DC 23 years old without SA for 5 tokens
ST AML 19 years old without SA for 6 tokens
DL 19 years old with defensive wall for 7 tokens
DL 29 years old with defensive wall for 3 tokens (was one of my players rotating out)
DR DMR MR 23 years old without SA for 3 tokens (also one of my players rotating out of the team)

So as I bought all of these (except for my own former players) for 1T we're looking at 24T profit in 4 days. At the start of the season it's a lot easier though to get even 5-6 tokens for mediocre players. So for example the DC 23 years old.

Again, the players you buy should ideally be younger than 21.
Also you have to look for certain positions that are easier to sell than others. I've made the following experience:

GK - Not recommended, sells poorly
Defenders - Double or triple position sells really good. Single position DC is solid, DL and DR only recommended with SA. DC/DMC combination is okay.
Midfielders - Stay away from single positions except MC. AML/AMC and so on sell really quick. Also things like DMR/MR/AMR don't sell too good.
ST - Sell really good, especially when they have any SA or are ST/AML and so on.


I hope this helped you a bit and I didn't forget important things. Also I'm quite far in the game on level 41. So all players cost the same and I have plenty of money to be spent. And the managers in the levels below me are also quite eager, which means, they probably spend more tokens than players in level 1-20.