1. Fully describe your knife: What is the make/model, materials (since many production knives are produced in different variations), dimensions (less important if it's a fairly common knife, very important if it's custom or something), and condition. The condition should tell people how much you've used it, not be something subjective like "Very Good." Don't sell knives as "NIB" or "LNIB" if you've used them or if you bought it second-hand and therefore can't know if the previous owner(s) used them, even if they were sold to you that way. It's better to be up front, i.e. "I did not use or carry this knife, and the previous owner said he did not use or carry this knife," or "I carried this knife and it has marks from my pocket (smoothing on the G10, snail trails on the titanium), but it has only been used to cut threads or open a few plastic bags."
2. Take accurate pictures of your knife that are clear, in focus, and in decent lighting. Take a picture of each side at a minimum, plus close-up pictures of any imperfections. Other common pictures are spine pictures of fixed-blade knives and pictures of the lockup and blade centering on a folding knife.
3. Determine your price. You are not required to use Paypal G&S by Bladeforums, but it is best practice, as you are engaging in a commercial transaction, and Paypal G&S provides the most protection for buyers and sellers alike. There are multiple sites where you can calculate what the G&S fees will be on a given transaction amount, or reverse-calculate how much you have to ask in order to get a specific net amount. My base price would always be (Cost I paid) +/- (Any loss or potential gain in price) + (Paypal fees to get the net desired amount) + (Cost of shipping, handling, and shipping materials). Put this as an all-inclusive price up front - buyers shouldn't need to calculate your prices for you.
3. Determine the conditions of your sale. It's nice to state clearly the terms of return/refund, but in general regardless of what you say, you're responsible for the knife getting to the buyer in the same condition you described it to them in. Here you may want to specify your means of shipping and alternate payment and shipping options, as well as whether you're willing to ship overseas. Shipping to a different country involves custom declarations forms with a declared value of the item you are importing. The receiver may have to pay import duties on this item. Many international buyers will want you to label the item a 'gift' or 'garden tool' or 'camping tool' and put a very low declared value on the item. I won't do that myself, as I am concerned that a low declared value may negatively impact a shipping insurance claim, but people will ask. Also international shipping may cost you more and not all places will have good package tracking once they leave the US. So you'll want to be up front about how you will handle international sales (or won't handle them).
4. Ship promptly. Same day or next day as payment is confirmed as received when possible. I tell people that I ship same- or next-day during the business week or on Saturday if payment is received and I am notified in time to do so.
5. Pack your knife with appropriate packing materials. Ship in a box, not an envelope or mailer bag. Pack the box with packing materials - bubble wrap, packing peanuts, even wadded-up plastic grocery bags or newspaper is perfectly fine. You've got enough packing material if you shake the sealed box and nothing shifts or rattles. I put bubble wrap around knives inside their own box or pouch as well as around the box or pouch in the shipping box. I use one of the Tyvek USPS Priority Mail bags to put a knife in (for the water resistance) and then pack it into the desired box with packing materials.
6. Tape the outside of the box. At a minimum, tape all the open or perforated edges. Best rule of thumb? You're shipping a knife, but it should require the receiver to already have a knife in order to get into the box.
