sam – it is called meiver dnahar simply because men don’t have ladies with perfume in their back pocket. walking into the store to buy has no shayachos to what he smells here. there he smells the smell here he is smelling her. it is not exceptionally common to walk by women with so much perfume, so by doing so you are supplying him with this issur, making this clearly meiver dnahara. he has no way of accessing this (not the smell but the combination) by himself. the lifnei eiver is not if perfume is for sale.
yitayningwut- i do not think the gemara is very clear that that din is lmaaseh. the earliest source i think is the peirush hamishnayos lrambam on shviis.