1) See what is on the daily deal site.
2) Check your research tool or completed listings and determine if you can sell the item at a profit.
3) Watch the shipping. Some sites charge a flat shipping fee no matter how many you order, other sites charge per item. So watch your cost on shipping.
I recently found these mini photo studios on a daily deal site for $9.95 with $5 shipping for the entire order. I bought 5 of them. I listed them in store inventory for $39.95 in my store. Here is my listing:
I have sold 3 of them for a profit of $18 each. Not bad for something you can buy online, have delivered to your door, and just slap a label on to reship. Here are the daily deal sites I use:
1 Sale a Day