It is possible the original owner was/is truthful and simply relied on the bike shop he used to catch any and all problems. If he is not a mechanically savvy owner, he may have asked them to look at it and figured the 150£ bill caught all the problems. It is also possible he wanted to make the bike just good enough for sale and so selected the bare minimum from a range of available repair options offered by the shop.
I ran into something like this when I purchased a used rental bike from a reputable pawn shop. The description said it had also been "fully serviced" but everything aside from the frame was just shot -- even the brake pads had worn to the point where the mounting studs had deeply scored the rims. No matter to me as all I wanted was the frame and the price was low for even just the frame so all good and it built into a fine bike, one of my favorites once I fitted it with a Thorn Sherpa Mk2 fork from SJS Cycles. However, the pawn shop had no idea about bikes to the extent the frame was listed as a "girls bike" because the top tube sloped slightly! It happens.
Given the overall cost and the make/model of the bike, my opinion is you received good value overall for a fully serviced/serviceable bike going forward. However, I can see the apparent deception still bothers you, so I think you were wise to contact the seller to see what he says. Ultimately, it might fall under eBay's classification for "Not as described -- wrong condition", but the ideal time to make a claim would have been after receipt but before paying for repairs, basing your claim on a written estimate of work needed before splashing out for repairs. If you were to receive a refund for the bike via eBay, it is likely you would be compensated only for the purchase price and not the additional repairs made. It might be helpful to consult eBay's UK policies (they vary by country):
https://pages.ebay.co.uk/ebay-money-back-guarantee/index.html Each situation varies. Perhaps the seller would be willing to compensate you in part or full for the unexpected repairs.
Best of luck, looking forward to any new developments.
Dan.