They'd likely still come out ahead if their cost on the item is ~$2.50 or lower, depending on their shipping costs. If I were shipping a 1-oz letter to Hong Kong,
ebay would take 50 cents, PayPal 45 cents and postage would be $1.15 (lower if I wanted to use discount postage). Add another 10 cents or so for an envelope and my total costs would be in the $2.20 ballpark, $2.50 if I have to also figure in an insertion fee. They certainly wouldn't make much, but they would still come out ahead if their product cost is low enough.
For things like small electronic pieces (cell phone cables and the like), sometimes the listing might state the seller is in Hong Kong, but the item still ends up shipping from somewhere in the US. At least that's what I've seen when purchasing items like that.