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I worked (and still do on and off) for a company that specialised in this. Without going into too much detail it's essentially one guy based full time out of an independent bike shop. In order to support his salary and wages for myself and one other mechanic (both part time) we were turning around 30+ bikes a month typically valued at £500 and upwards on top of regular repair work.
In order to make enough money to make it viable as a business you need to be dealing in 'high end' bikes (think margins). The trouble with this is the people who have the means to buy what you're selling will probably want to try it beforehand. This is especially the case if you haven't got a brand/reputation behind you. So you need to be available for viewings or be willing to post. Then you're into the realm of postal returns. Which are a nightmare.
TL;DR It's doable, but difficult.
What's the general consensus on buying-tidying-selling bikes as a way of supplementing a salary and cutting down office hours? @Dogtemple mentioned it previously and i had a brief chat with @littleK a while back about this, keen to find out what experiences people have had with this.
I don't earn a high salary so i wouldn't need to make much to make it work but as i'm someone that isn't very good at switching off i'd be concerned that mentally it could be a bit much (thinking about builds, listings, checking pm's etc).