Billdelong's picture would confirm a situation like what you first thought, the ESC is frying receivers by delivering an unrestricted 16.56V from the battery, instead of reducing it to 6V-8.4V as it would if operating normally. This function is known as a 'battery eliminator circuit' because it allows the steering servo and electronics to function from the main battery power, rather from an addition 'receiver pack' battery.

So options could include buying and adding in an external BEC to regulate the voltage. You would need to remove the one of the power connections from the plug coming out of the ESC for this to work. You could do a similar thing by removing a power connection from the ESC plug and adding in a receiver pack to power the receiver and servo that way.

If the ESC came with any kind of program card or options to program using the esc buttons, you could look for a setting to adjust the BEC voltage... but it would be unusual for a functioning car ESC to put out such a high voltage to the receiver no matter the setting. You could still try messaging the seller about the problem, and even ebay/paypal dispute resolution if the item was not described as faulty in the ad. Given the problems you have already had, buy another ESC and check the output with a multimeter first!