using digits 1-9 only once, multiply a 3 digit by a 2 digit number and get a 4 digit result

12*345=4140?

looks ok to me.

the answer would have to be 6789 or some order of those digits if you used the others in the question. digits 1-9 used only once, no zeroes.

The digits can be used in any order. We have gotten 8 out of 9 digits in one problem 456x13=5928 (2 fives and no 7). Will keep trying.