Keep it.
Price and value don't always correlate. Example ...
I run out of petrol taking a friend to hospital. You pull over to see if I'm okay. You offer to sell me your spare can of petrol for �100. At that moment, to me, you petrol is worth �100. I buy it.
Afterwards, do you feel guilty? No. We both got what we wanted from the deal.
I could have walked to a petrol station. You eBay seller could have done his homework.
But he wanted some quick cash, and he got it.
He's happy cos he got his quick cash.
You're happy cos you got a bargain.
Do you remember the guy a few years back who saw the old plate at a car boot sale, knew what it was, bought it for 50p and sold it for �19,000? These things happen.