Pione Sisto. Winger.
Shaqiri. Winger.
Mario Gotze. AMC.
Roberto Pereryra. CM.
Sergi Roberto. CM.
Eder Alvarez Balanta. CB.
Daniel Rugani. CB.
Lucas Romero. CM.
Geoffrey Kondogbia. CM/DM.
Gaston Gil Romero. CM.
Isco CM.
Doria CB.
Caulker CB.
Eric Durm FB/WB.
Luciano Vietto ST.
Jeronimo Rulli GK.
Jan Oblak GK.
Mayke FB/WB.
Lucas Silva DM.
Joshua Kimmich DM/CM.
And many more. Usually your players inflate in value after having a good statistical season and/or your team winning some trophies - ie league, cl, fa cup, etc. Basically buy good players, have a good assistant coach to train them, and after a couple seasons they should increase in value, sometimes double/triple. And then during the summer, put them on sale for 2x/3x their market value and hope that one of the big budget teams gives you a crazy offer.
Examples:
Bought Kondogbia for 25m from Monaco. Later sold for 53m to Man City.
Bought Gotze for 12.5m from Bayern. Sold back for 53m to Bayern.
Bought Gaston Gil Romero for 3.1m from Estudiante. Sold to Juventus for 29m.
Bought Lucas Moura for 6.5m from PSG. Sold to Dortmund for 25m.
Got Shaqiri on a free. Three seasons later he is worth 32.5m.
Got Roberto Pereyra on a free. Two seasons later he is worth 33.5m.
Got Rugani for 6.5m. A few seasons later he is worth 20m.
Got Alex Telles for 7.75m. Just one season later he's worth 18.5m.
Some of these guys you need to wait till they are unhappy and scoop them up at a bargain price. Some of these players will be transfer listed. And some require scouting to find but are available for low prices because the parent club doesn't give them many appearances or has excess depth.
With good scouting, training, and a successful season, the value of your players should continue to grow.
And at that point you have the luxury of selling on the players who are surplus to needs but will give your club a healthy cash injection upon sale.