Realistically, you put yourself in a very bad position by hauling for others whether you charge money or not.
If you don’t charge money, you can still be left holding the bag if someone’s horse damages or makes a mess of your trailer. If that person’s horse get’s injured while hauling or you are in an accident where the horse gets injured, you could be held responsible for vet bills if the owner perceived that the accident was somehow your fault. Loading and unloading horses is a common situation in which people get injured and you are assuming at least some personal risk by transporting an unfamiliar horse. If you get in an accident or have a breakdown, that is one more horse that you have to worry about being responsible for. When you trailer for other people, there is also an element of chaos as your usual loading/unloading routine may be disrupted and an unfamiliar horse can behave unpredictably and those things can have safety repercussions.
If you charge money, there are some serious insurance implications. For example, regular car insurance does not include coverage for commercial hauling. This means that if you were in an accident, your insurance could say, sorry, we aren’t covering any damages to your vehicle or the damages/hospital bills of the other party. You also technically may need a CDL–something to check on. If you charge money, you also are clearly open to liability for anything that should happen to the horses in your care during the transportation process.
Also, if you add up what it costs you to own, maintain, insure, and fuel up your rig, and calculate out a per-trip or per-mile hauling cost, a number that fairly reimburses you for your time, inconvenience, AND the aforementioned expenses–it’s a pretty big number. In fact, it’s about what the commercial haulers charge.
Considering that there are many commercial haulers out there, and that the cost for commercial hauling is pretty much what it actually costs to provide the service, and the enormous responsibility/liability of transporting someone else’s valuable 1200 lb. animal, no one should ever feel guilty for refusing to trailer someone else’s horse.