I've renovated one bike and built up another from a frame, largely using
eBay vintage components. I did both for myself and spent about £250 apiece (excluding Brooks saddles), which is nowhere near what I'd get if I tried to resell them.
I also got an old rigid MTB with stuck gears for £30 which only needed the indexed shifters cleaned and relubed, and I expect it would sell for more now. I'm going to replace cables throughout, new brake blocks, new pedals, new saddle and keep it for myself - but if I added the cost of those parts I wouldn't get my money back by selling.
To make money on a bike clean-up, I really do think it needs to be little more than cleaning and adjusting, and new cables and brake blocks at the most - and then only if you got it for a real steal in the first place.
I think there's a better chance of making money doing the opposite - buying up bikes and stripping them for parts. Decent old derailleurs can easily go for £20, and if the bike has good brand brakes, brake levers, seat post, stem, cranks... the value of parts can soon add up. Add in a recognised frame maker, and I reckon you could do well, though you'd almost certainly have to dispose of some unsaleable old parts too.
I regularly see vintage steel bikes going for under £100 with components that I think would sell for twice the bike price or more.