I've had a number of very successful MTB flips. You just have to be realistic. Most of them have been 10 or $20 lower end LBS bikes from the early 90s that were in like new condition. They are super easy to sell for $50 or $60, and it's worth it to make $40 for not doing any work other than posting a craigslist ad. It at least covers the cost of gas and maybe breakfast while out yard sailing.

The other bikes have been ones with unique parts that are worth money on their own. I have many examples of buying an otherwise unremarkable MTB with one or two rare parts, putting them on ebay for an obscene amount of money, putting less rare replacement parts on the bike from my parts stash, and making an easy $150 or $250 with very mimal effort.