IANAL, but I suspect the sales of such a converter (without UL certification) is in no way illegal. Heck, people sell stuff like 2.4GHz or 27MHz RF amplifiers or non-cell-blocked RF receivers all the time, and everyone knows darned well that 99% of the buyers are going to be *using* such equipment in an illegal manual, yet the sellers are often left alone if they do a good enough job of describing their goods as "test equipment" or similar. In your case, there's not even any particular attractoin to use such a device in an illegal manner.
That being said, if someone buys your product, it starts on fire and burns their house down, you almost certainly will be sued these days. You can add as many disclaimers as you want to the product, but that won't prevent your loss of time and money when you're forced to win your case in court. I would guess that about the only effective way to prevent much of this would be to get a *written* agreement from potential customers that you're providing no guarantees, the product is not UL listed, etc., since they it should be a pretty much open-and-shut court case should the issue ever arise (unlike "click-through" or shrinkwrap licenses, signed agreements hold a lot more weight).
Alternatively, figure out how to set up a real corporation somehow, so that -- worst case -- the corporation is sued out of existance, but your personal well-being is largely untouchable. (The "corporate veil" and all that -- unless *gross* negligence can be demonstrated, you're safe.)
---Joel