The 'smart" way to proceed would be to purchase an initial
supply of say, 500 units from a company that can ship up to
5,000 more on short notice if necessary (don't make the mistake
of buying the last 500 they have). With an investment of $500
in stock, place your ads and wait for orders to come in.
It is also possible to have the items drop-shipped and avoid
the investment in stock. Under this system, you fill out a
shipping label for each individual order (with your return
address) and mail it along with the wholesale price and all
the P & H charge to the supplier, who mails the product direct
to your customer as your "warehouse."
The advantages of drop-shipping are that you don't need to
invest in merchandise until it is sold and that you don't
run out.
The disadvantages are that the merchandise costs much more
(no volume price breaks(, it takes extra time and postage to
forward the orders to the supplier, and that the supplier gets
all the postage and handling fee (usually part is profit).
Most mail order dealers stock their good sellers and use drop-
shipments for slow movers and new, untested items.