First of, “Handeln Mit” is two words, so if anything it would mean “deal with”. The actual meaning would depend on the context of how it was used in a sentence. As a stand alone phrase I'd probably translate it as “to do business with”.
There is only one other .com registered that uses a similar combination which is “Gewinn Mit”. There are plenty of more common “mit” phrases that are unregistered.
That being said, tradewith.com is the best closest English translation that makes a nice domain and was registered in 1999. There over a thousand “with” suffix .coms registered. Perhaps the name will have value as the German speaking market grows. But you will have to hang on to it for a while if you want a chance to see it worth anything.