Most guys who are kit collectors or buying older kits will want you to "double box" the kit for shipping to protect the box itself from damage. I will also pack some newspaper within the kit itself to keep the bits from moving around too much. Most kits back in the day were just taped shut, labeled and dropped on the shipper's truck, received plenty from Tower Hobbies like that.
I have access to large sheets of scrap cardboard, but good sources are appliance stores, bike shops, furniture stores, etc. The easiest way I've found to double box is to lay the kit the long way with the flutes of the cardboard, and trim the sheet to the exact length of the box. Score the inside of the cardboard with a blunt object down the length of the box and "roll" the box up on it's side. Score, roll, score, roll. You can then trim the cardboard and have a nice cardboard "sleeve" to protect the kit. I tape that up well to itself, make sure you snug it nice and tight to the box, (don't tape it to the kit box or your buyer might have a coronary) then make ends for the sleeve from scrap and tape in place. Make your life easier and get a tape gun, the $20 or so is well worth it.
The most reasonable method to ship will be USPS Standard Post, but it takes forever. I had a kit shipped to me, it left CA on the 13th and won't get to me until tomorrow the 27th. You'll find that in many instances Priority Mail is a dollar or two more and it's at the buyer's doorstep in three days or less.
Before you commit to a shipped price, be sure you use the calculator on USPS or the Paypal site.
You'll save some money shipping online with them, tracking is included on all shipments and you can print labels right from your home computer. Sure beats standing in line at the post office and paying extra for things. A lot of guys will offer things for a shipped price within the lower 48 states only, which is kinda rude as there are
two other states in the Union. It might cost a couple dollars more to get it there, but it's nothing like international shipping.
If you're using Paypal, you can ship right from the payment page. If the item is $200 or over, be sure to send it direct signature required, this covers your butt with the Seller Protection in case a buyer decides to file a claim. Paypal states $250, I use it at $200. Fedex or UPS are also an option, but don't use their site, you can ship UPS through Paypal, Fedex through eBay if you list the kits at auction.
Hope this helps...