I got a previous mare that way. I was helping train her for the owner that had bought her as a resale project to learn some things. The mare turned out to be a bit looney but liked her so when I was done with the training I was doing, I asked if I could keep riding her. The owner said yes, then then owner decided that while she is a resale project, she doesn’t have a lot of interest and could end up in a dangerous situation. In the mean time, she had another horse that needed training. So I put a couple of hundred down and worked off the rest of the price ($1500 to my recollection) in training rides. The owner was also pregnant at the time so I could keep the mare there for free if I fed and cleaned stalls (4 horses) 6 days a week. It worked out very well for us.
Currently, I trade work for board and sometimes we have to have a discussion on what that means - there is no written contract so I am basically salaried and sometimes we need to reassess value. Overall, though, it works out well.