I was a long-time active member in Find A Grave and documented two local historic cemeteries there.
As has been stated, they are wholly owned by Ancestry although by agreement management of Find A Grave is by the original owner and creator of the site and it remains free for the user.
As to selling info, it's a little complicated but in short if you post on Find A Grave you give them the legal right to publish whatever you posted (e.g. photos). So, if you are one who is concerned about copyrights, make sure you understand that by participating there you have given them rights. When Ancestry took over a number of older members removed all their memorials or photos for that reason. So while they aren't likely going to sell your work directly they are marketing it in a way.
I was never worried about photos I took of markers but I did remove a few family photos. I still think it's a good site in general and a great resource for genealogists (although as with anything open to the public without review there are many errors). I received a number of thank yous from people who found a lost ancestor through one of my memorials and I found a number of my folk when I was researching the family.