It's legal to sell kits. Don't assemble it at all. Assuming you have a disclaimer of liability statement prominently displayed within the purchasing process, it is the person who builds the device who assumes liability.
Once you start preassembling them, UL listing or not it is you who assumes liability, a good insurance policy should be in place by that point.
Nothing is bulletproof, nor do you have any reasonable guarantee that even if your design is perfect, that someone won't improperly assemble it or spill a bottle of liquid inside, throw it in the bathtub, knock it off a shelf and then try to use it attached to a kite during an electrical storm.
All you can do in that regard is put thought into it's ruggedness and test it as much as your conscience demands. Spontaneous failure and fire can usually be prevented with basic steps like a good strain relief at the chassis, fuse(s), adequate clearance between parts and cementing down anything prone to flop around due to it's height vs weight (though in this latter case, it's part of the instructions for the assembler). Test it in high ambient temperature conditions, power cycle the heck out of it. Drop it on the floor a few times and see what goes wrong. At that point you have exceeded due diligence, most finished products you buy won't survive these conditions.
Make sure you clearly state there are no refunds once assembly has begun and emphasize safety during construction and use. At that point each individual customer assumes responsibility for their ability to understand the circuit and build it properly, but if you don't have an EE degree it would be good to have an EE validate it in writing just in case anyone ever tried to sue you for their own mistakes.