There are no objective criteria to describe a suspicious transfer.
You must keep in mind that you may never ever pay an extremely higher amount for a player than his market value. You can always check the market value by using transfer compare.
In the past it was allowed to 'exchange' players (to make an agreement with another user to make the same extreme high bid on a player of the other person so you could 'exchange' the two players). This is not allowed anymore.
If someone you don't know makes an extremely high bid on one of your players, be smart enough to report it to your local GM. They will find out anyway and you will lose the money that was payed to much anyway, but if you don't report it to your local GM's they could see it as cheating on your part and sanction you about it.
So always report transfers that look suspicious to you by contacting your local GM.