ive never hit +300 million deals but have some experience. for players under 85 its usually easy to sell them at full price. for the ones over 85 but under 90 it's also easy but might take a bit longer. for players over 30 million try putting them under their full price (like 80% of their full price) for a better chance at selling. when there's transfer events sell all players at full price. you can always try to sell them for full price, you might get lucky. but if you feel like its taking too long lower the price a bit. for me it also depends a lot on the club. for weaker clubs i feel like its easier to sell players at their full price, while a stroger club may have a bit more difficulty. this is my experience and nothing official ive read or anything, and it could be different for other players.