No. That is not cheating.
If you put a player on sale and you're worried because the offers are lower than expected, it's perfectly legit to bid on him.
But don't forget: you will pay 100% of your offer, but receive only 80-97% of it (depending on the player history), so you will lose 3-20% of the sale price. Worst yet, if you actually buy back your player the % on the future sale will revert back to 80%, so you really have to be careful on your choice.