As Ray has said, wireless systems can be problematic.
There needs to be a clear line of sight from camera to receiver to get the best reception and they are influenced by passing vehicles, sudden loss of signal and not very good all round.
Of course , as per normal, you only get what you pay for, so a £70 system is not going to be up to much, also,l buying from EBay, is the seller just a box seller or does he have a good after sales system should anything be wrong with it?
Wired systems are far superior, but so are the prices, and th every best wired systems are the Waeco systems which is part of the Dometic Group so backup is very good.