I have a line job at one of the nicest FBOs you'll find anywhere. Red carpet, leather this, marble that, a few bizjets and the occasional airtanker or heavy as customers, people love coming to it. It definitely fills a niche that no one else -- at least not on our field -- can, that is being a really nice "golden age" FBO. Still, I can't conceive of how it makes any profit. It's been open quite a few years and if it hasn't broken even yet, it wouldn't surprise me.
An FBO is the kind of business that works great for someone who already has money. LOTS of it. If you can afford to lose a few hundred G's, go have yourself a ball selling as much fuel as you can to offset the loss and delay your capsizing.
On the other hand, I'm stepping into the business realm myself soon with a restaurant franchise. It's definitely not a "pet business" that I can afford to watch sink, shrug, and walk away from. I'm working to get my plan for it airtight as possible, and I'm still shaking in my boots.
Unfortunately, 99% of startup FBOs will do poorly since most anybody with a) a passion for aviation, b) an insane amount of money, and c) a shred of business sense would wisely put their time and money into more profitable endeavors.
MFR