Scouts, if they are within 10 skill points of world class quality and are aged 22 and are versatile with a good SA then it's definitely worth investing in them. I bought a scout last season, 22y/o -AMC/AMR -Corner Specialist, he was 6*(34) and at the end of the season was an 8*(40) player. Now he's 7*(41) and will probably be 6*(44) in the 3rd season, 6*(46) in the 4th season, 5*(47) in the 5th season. In the sixth season since buying him, he will still be in my squad but as a 4* reserve player (and in my hall of fame because he's amazing). So investing in a good, young scout who is close to world class quality is definitely worth the 50 tokens as you can keep him for many seasons!

Now, as for auctions. It really depends how much you are willing to go for some players. I tend to buy 5* youngsters from the auction list as they train fast and will be 6* players within a few days. If I wanted a 5* 30+ year old player who I'm going to ditch at the end of the season, I'll probably only be prepared to buy him for 2-3 tokens. Buying players who are close to scout quality is definitely what you're aiming for. You can also use their ages as a guide for how much to spend on them. Personally, I'd only bid over 15 tokens on a player if he's 18-21, a fast trainer and very close to the next star quality (SA helps too). Through natural growth and training, he could stay for 2-3 seasons. But for a lot of players, I would not spend more than 7 tokens on a player that you're only going to keep for one season and then end up benching him next season, it's a waste and you can invest in players who can stay for longer. If you're really lucky you can get players for 1 or 2 tokens, depending on the time of day... Do this and you've found yourself a bargain.

And I know nothing about negotiations so I can't help there, sorry!