I started a community forum focusing on MMA trading cards when they first started being made.
I thought it would make money on transactions, but ended up being the main hang out, kinda like this site for carpet cleaners.
I made all my money on affiliate marketing clicks. In it's hayday, about $1000/ month.
Then Facebook just killed it, as it was easier for groups to start and posting pictures was easier and posting photos was easier.
And Topps started over producing once they caught on, diluting the value.
I made enough money the first 2 years to pay for the building of the site, and I own the holy grails of MMA trading cards, kinda like owing the michael jordan rookies of Basketball cards or the Micky Mantle rookie of Baseball.
The best flip I ever made was on a Jon Jones Rookie Red Autograph (only 25 made) and bought 4 of them for $250 each. I sold one for $2500, and still own the other 3.
I have a few of the 1/1 serial numbered rookies from the first release of the biggest names in MMA. (St Pierre, Anderson Silva, Chuck Liddell, BJ Penn, etc)
I keep the site up, but no more money goes into it other than monthly hosting, and I buy all my ebay purchases and make about 5% commission, so that alone is worth keeping it alive.
But it seems only by the sweat of my brow, do I ever profit.