Not just the cost of the miners you purchase. Escrow shipping as well.
A thread like this:
https://bitcointalk.org/index.php?topic=2757777.0Seems normal, and the buyer says he accepts escrow - which he does... but requests shipping charges upfront. He will charge you for shipping per unit, and if you purchase more than one unit, that can add up.
Once you pay the shipping, and tie up your bitcoin in escrow, he moves on and stops responding. If you use a service like EscrowMyBits, your BTC is tied up for 7 days and you pay the 1% escrow fee. Additionally, by locking the thread, he can keep out bad reviews.
Scammers abound, but remember: escrow EVERYTHING. Do not pay ANYTHING up front.