That depends on the circumstances. By it from a stranger in a pub at a knockdown price the court will almost inevitably take the view that a "person of reasonable firmness" would reasonably expect it to be a bit fishy.
Claiming ignorance and asking no questions won't help you - the law experts a reasonable degree of due diligence.
When I bought my first ebike it was from an advert in the local chippie
I had been looking at them online but when I saw the price I decided "NO WAY!!!" as they were over £1000 in 2011
then I saw an advert for a Powacycle Salisbury for £220 - in felt pen on the notice board in the chippie - the very bike I had been looking at that morning online!
So I rang up and he lived a short walk away so I went round
anyway - it all looked ON - he had the manuals and everything and the serial number was intact
he said it just didn't have enough power for him as he wasn't a light bloke - and gave me a "look"
and he had a Giant ebike parked next to it that he said was better for him
but I went for a test ride
and handed over the cash - he insisted on cash only
but I did wonder - but it looked OK and I knew that the Powacycle was only 180W when the limit at the time was 200W so it made sense
I did write down all the reasons that I thought it was OK when I got home - just in case!
so it could have been dodgy