Generally, the line of morality is drawn by establishing a connection with either side of the actual sale. Dropshipping does not involve factual purchase of the product and its resale for a markup. The dropshipper should either be in contractual agreement with the producer to promote and sell specific items, or to supply him en masse on demand, in most of which cases a markup or resale prohibition is specified (see Microsoft Volume Licensing). The other option is that the dropshipper is appointed by the customer to purchase a generic item, say "luxury watch", in which case the markup margin for finding a suitable item should be known to the customer.

As per Islam's rulings, one should not sell what he doesn't own. More rigorous treatment of the issue could be found in the religion's teachings beyond my humble input above.