You may not need to make frequent runs to the post office. eBay has collected a great deal of data about what has sold on their site over the last 20 years and will give you an estimate based on the item you’re selling and your zip code. If eBay doesn’t recognize what you’re selling (for example, a collection of kit, photoetch, and book), it’ll require you to enter the weight of the package. Investing in a small postal scale (about $20) will let you do that at home, using the USPS web site to determine the actual postage costs.
STORM is correct above about prices. I’ve posted this link several times for others over the last few weeks offering advice to folks like you. My suggestion is to start most auctions at 99 cents and let it ride.
Scale Model Soup