Electronic payment is far preferred, with PayPal being the dominant one. PayPal allows you to send money in two ways, one, via Goods and Services, is intended for commercial exchanges, and the other, Friends and Family, is meant for non-commercial transfers like splitting rent, repaying someone for dinner, or giving a gift. PayPal charges seller fees for G&S. There are no fees for F&F if paid by bank transfer, but the sender pays fees for F&F if the fund source is a credit card.
F&F is widely used to sell knives against PayPal's Terms of Service because it allows the seller to evade PayPal's fees and because a tax law change two years ago now requires payment processing companies like PayPal to issue and report 1099 forms each year for people receiving G&S payments. Theoretically we were always supposed to pay taxes on net income received like this, but no one did it. It's a hassle because it requires you to keep a log of your collection inventory acquisition dates and cost basis so you can file the transactions on your taxes. Falling to keep track would mean the feds would assume ALL the reported income was net profit and would dramatically increase the tax burden from it, since you were not able to document the cost of goods sold.
However, using F&F to accept payments for knives sold is tax evasion and also against PayPal's ToS, hence, it's also common to see people who do not accept PayPal, as many have had their accounts shut down for selling goods and requesting payment by F&F.
Taking payment by G&S is not entirely without risk, though, as PayPal has vague policies that are sometimes arbitrarily enforced against transactions where knives are sold. So it's a matter of something that is DEFINITELY against their policy (misuse of F&F) versus something that is not against their policy but which could be arbitrarily treated as though it was with no explanation or recourse. However, they're are also plenty of examples of sellers who had to resort to seller protection and were covered when someone who had used F&F would just be out of luck.
That being said, personally I price and request for G&S, but if people pay me by F&F, I don't refund and have them pay by G&S, I just ship them their knife. It's only against policy to request F&F payment for commercial transactions. If people pay that way even though I said to do it by G&S, I feel like I did my fair part.
In terms of after the sale, you need to ship promptly. Ideally you'd include in the sale post when you can ship and how. This should usually be same or next day of payment clearing, but if you're going out of town or something, as long as you're up front in advance about it, it's fine.
You can get free Priority Mail boxes from the USPS sent directly to your house at no cost. The small flat rate boxes generally work well for most folders unless the knife box is too big (small flat rate box is 1-5/8" tall on the shortest dimension). Koenigs don't fit, and Hinderers are a bit tight. But what I actually prefer now is the small priority box. Not the small flat RATE box, but the 1096L box that is 9-7/16" x 6-7/16" x 2-3/16".
The reason is that these will comfortably fit almost all folders in their factory box and more importantly, they will go Priority Mail instead of flat rate, which means the actual package weight will go on your receipt. [EDIT: This is under dispute, I may be wrong] Since weight doesn't matter for flat rate, it doesn't go on when you use a flat rate box. That's useful in case a buyer claims he received an empty box, because you can prove you sent a box that weighed enough to have the knife in it, and each post office scan actually has the package weight at scan, although the Post Office has to find that info, you can't. But if they have to investigate a lost package, the weight tracking can prove it was lost in transit and not after delivery.
You also need to insure for the full replacement value unless you are willing to eat the cost of refunding the buyer in the event it's lost or stolen before receipt. If it's lost, you refund immediately and hash out out with the insurance, don't make the buyer wait for the insurance claim, which can take a while.
The absolute best insurance, though, is securely taping all edges of the box and padding heavily with bubble wrap or packing paper.