It depends on the item, what it is worth, how much they have sold for, how much it weighs, how much it will cost to ship, etc.
Sometimes I use it to help market an item for more than it is worth.
For instance, I bought 6 toy train switches in a box of cheap track for $2. The switches in completed sales were not selling at 4.99 to 5.99 plus shipping. Since I know the toy train market, I took the few pieces off of the switches and sold them as used original repair parts. I sold 2 nuts for 6.99 with free shipping. Postage was 65 cents a package. I had about 6 sets of those nuts.
(FWIW, I sold the switches at the flea market for $4 each after I stripped the nuts off of them, and sold packs of 10 pieces of track for $4 each, and the nuts on ebay for 6.99. I made about $82 total from that box that other sellers would have said was worthless.)
The free shipping added a bonus to the buyer, strengthening the value of my item, and barely cost me a thing to ship.