Sellers of clothing are permitted to put on four-cornered garments to which tzitzis are not attached, because their intent is not to derive benefit from wearing them, but to show their size to their clients. It is forbidden, however, to wear a garment without tzitzis to avoid paying customs duty which is charged for merchandise, but not for one's personal garments. At such a time one derives benefit from actually wearing the garment, and this is his intention, for in this way he evades customs duty.
(Shulchan Aruch HaRav 191:3)