You could use food-grade linseed oil, but I have heard of people taking a bad reaction to it (vomiting).
Unlike tung and boiled linseed oil, walnut oil can go on the bowl staight from the bottle, and contains no petroleum based solvent, drying agents, varnish, etc. It also tastes good on the salad.
Careful with vegitable oils, they can go rancid in time.
Is your wood food safe? Rock maple and beech are supposed to be traditional woods used in food contact. I've heard that maple actually contains a natural ability to fight bacteria, at least better than some others.
Anything other than a label saying something to the effect of "pure" or "100% tung oil" are not pure tung oil, but just some kind of wiping varnish that may or may not actually contain tung oil. Pure tung oil is just that and nothing else. I use it extensively, both alone and as part of a homemade varnish concoction, on woodturnings.
Most products that claim to be "salad bowl finish" are said to contain ingredients that once fully and properly cured, are supposed to be considered safe by the FDA. In actuality, all surface finishes other than shellac cure to some kind of a plastic film. Don't use shellac, even though it is food safe, acidic food materials like salad dressing can damage shellac. Salad bowl finishes are a variety of a wiping varnish.
Many will suggest using walnut oil as the finish. It cures, eventually. It's a slow curing process, though (I've never used it, but I would give any finish several weeks, maybe a full month, to cure before putting into use for food), but is food safe and available at grocery stores. I'm not sure how people who are allergic to nuts would be affected by this oil finish, though.
If anyone suggests mineral oil, beware that mineral oil does not ever cure, and will be washed off by dishwashing detergents, so you will have to reapply it often. Some people do use mineral oil, though, especially on butcher blocks, etc.
To sell them you have to first of all make them appealing to the buyer. This is down to choice of wood, shape of the bowl and the finish you get from the tools and sanding, in other words did you make a good job of the bowl.
The other sort of finish, the sort you apply after you have finished is a matter of choice. I would go for an oil finish using an oil I wouldn't mind having on my food, so vegetable oils such as sunflower, olive etc., and in my case nut oils. But for selling I'd stay away from nut oils as there are more and more allergic reactions to these. Mineral oil is safe to use not that I'd want it on my food. What you have to be mindful of is that whatever you put on it the bowl is not going to look in showroom condition for very long. You can suggest to the customer that the bowl will only need a wipe over with a damp cloth and a re-oiling to maintain it but you know that it will probably get left overnight with food in it then dunked into hot water to wash it. Re-oiled? Yeah sure. My point is the finish you start with is just that a starting point, the long term care of the bowl is what will determine how well it looks year after year. Make 'em good, sell 'em none too cheap and maybe the customer will value them enough to give them the care you would if they were your own.