Kevin
The sell / buy back strategy is only really worth doing if you have bought high level drivers from the market with very high wage demands (£1m +). Each time a contract renews the wages rise by 10%, the sell / buy back strategy can avoid the 10% wage rise. A word of warning... Be careful you don't trip up by selling a driver with a higher level than your team because you will be unable to buy back.
Most managers don't buy high level drivers because it is so much more cost effective to use the Youth Academy. However, the above strategy can be effective in controlling staff (CD / TD / Dr) wages when you get up to high levels.
Two more words of advice:
If you're driver levelled up during his contract he becomes more expensive, too. Even if you use the sell/buy back strategy.
Another point is that drivers bought via the transfermarket have only a contract length of 20 races as compared to extending a driver's contract, which will be a further 50 races. You have to calculate if that option is still cheaper for you, considering the amount of tokens you have to spend each time you buy the driver back plus the cash you also lose during this transaction. So perhaps it's a better idea to extend the driver's contract after the 1st 19 races and sell/buy him back at the end of his 1 contract extension, where another 10% raise would become due.