It depends on many variables, but here's my experience.

If you can get the bike for next to nothing and spend the minimum possible time getting it working, then you can make a little bit of money. Generally, I get bikes for £10 and sell them for £50. Some require only air in the tyres and a wipe down with a cloth. Others require replacement parts or significant repair. Sometimes it's not until you get a bike home that you discover the seat stay has completely detached from the seat tube (happened to me last week). Occasionally you'll find a £10 bike you can sell for £150. Occasionally you'll find a £10 bike you should be able to sell easily and have it sitting around for months on end waiting for a buyer.

If you decide to do this, you can expect 2 things: you'll have tons of bikes and parts cluttering up what space you have available. You'll spend more time working on them than you realise, and you won't make lots of money. Especially if you can't find a cheap source of replacement parts, tubes and tyres.

Last thing to consider is the bike market in your area. Are people buying used bikes? How much do they sell for? What's popular at the moment? For example around here retro mountain bikes are on the up, and vintage roadies are on the way down. Leisure/ hybrid bikes always sell quicker than I expect and I'm surprised how many people want one or ride them. Also, the 'pub bike' seems to be a thing. A BSO they can prop up outside the pub without fear of theft while they get a few beers in them.

Basically, if you're looking for a hobby that pays for itself and occasionally funds purchases for the bikes you ride, then go for it!