Is it legal to sell new...

Yes - you can sell what you want.

Resell the odd item because you decide you don't like it after all or because the fit isn't as good as you thought it would be and it is easier to resell it than send it back - no problem.
Include an odd item in an order for a friend or work colleague - again no problem.

Buy a dozen items of the same thing for reselling, or, worse, solicit orders from others by "advertising" - you are trading and it will probably be much, much more trouble than it is worth, whether you go "legit" or do it in the black.

Resell the odd item because you decide you don't like it after all or because the fit isn't as good as you thought it would be and it is easier to resell it than send it back - no problem.
Include an odd item in an order for a friend or work colleague - again no problem.

Buy a dozen items of the same thing for reselling, or, worse, solicit orders from others by "advertising" - you are trading and it will probably be much, much more trouble than it is worth, whether you go "legit" or do it in the black.