Physically, panels will fit into any design of frame molding. The panel without mat is usually never thicker than, say, a watercolor + mat + backing.
Buyers will do what they want including gluing string to the back to hang it with, but most will probably frame it in some way.
If you want to save your buyer from the expense of custom framing, you can paint on a standard size painting panel in your area (here in the US, 8 x 10 in., 9 x 12 in., 11 x 14 in. and so on).