I think the vast majority of people who have nice bikes an buy/sell a lot on here do a variety of the above. Find/split a cheap bike, look in the wanted sections to see if people want stuff, buy/sell on ebay, give stuff away etc.
I, albeit with a very small monthly budget, have now got 3 bikes which I never thought I'd have for very little monthly outlay. I really do think that maintaning a good presence on here and contributing to the site as a whole will enable you to build those special bikes much easier than just buying and selling for a profit. You just don't get offered stuff either openly or via a sly PM if you are a serial flipper for profit.
If you look out for people, help them out where you can, sell your stuff for reasonable prices to people who really need them, make an effort to contribute then you'll always find the stuff you want or need, and have some fun along the way. Flipping the odd eBay bike will be in that mix too and there's nothing wrong with that.
Take the example of the GT Borrego I had, yes I bought it as a full bike intending to restore it but it wasn't financially viable. So I sold the bits on to people who really needed them and gave the frame away to someone who will build it up properly and use it. Everyone wins - yes it sucks the bike wasn't kept together but it's out there somewhere being enjoyed in small aprts by a few people.
One of the greatest joys I can think of is rescuing bikes from all the 2nd hand shops and dodgy ebay adverts in West Yorks. All the bikes I've bought that way I either still have or have been cleaned, checked, split for parts and passed on (sometimes for a profit, although never very much) to someone who'll cherish it. They won't be used by someone who is just buying it to lock up outside the shops and gently rust away.
