When selling on eBay it is tempting to list an item and worry about how to ship it later. That is fatal. You should always have your shipping materials ready. For example, have the box, the tape, and the padding that will go around the item standing by, so that if you get the sale, you can quickly box the item and ship it.
One time I was selling an unusual item for me, because I was used to selling books. It was an original painting done for the cover of a romance novel. Painted on a canvas-covered board, it was rather large, long and flat. I made a nice sale, but then, never having packaged such an item, it took me a few days to figure out where to even get the large cardboard sheets necessary to protect the item in the mail. Fortunately, the buyer understood and things went well, but I could have gotten negative feedback for that delay. FTL
When I sell books, I put them in the box before even listing them on eBay, just to make sure that I have a box big enough and strong enough to carry them. I don’t want to be running around town, trying Office Max or Wal-Mart for the correct size, or going to the local liquor stores or grocery stores for box handouts.
Sometimes it is hard to find a box small enough for the few books that someone buys. I solve that problem by cutting down a larger box. I am merciless when doing this. The only alternative is making a separate trip to the store, wasting time. I get out my knife and slit the box at the seams, above the books sitting in the bottom. Then I just fold down these now extended flaps over the books. The extra cardboard that folds over from the long flaps makes for good reinforcement.
There is another great benefit for having your packaging done before the sale is made. You can weigh the item and know exactly how much it will cost to ship. Ambiguity in this area can get you negative feedback, and if you estimate the shipping without knowing the weight of the packaging, you could be losing a substantial amount of money over the course of a number of sales, or the customer might complain that your quote was lower than the actual bill that you send.
If you sell similar items all the time, you will get very good at knowing how to package and how much the shipping will cost; it is when you list an item that is new to you that you might run into trouble.
Take the extra time at the start so that you can iron out the problems before the customer has to pay the price in delays.