Online auction services, especially eBay have made it possible for any collector to become a stamp dealer. Get a catalog of your area, determine the catalog number, decide on a price of the item you're selling, list your item. Simple. In fact, you don't even need a catalog. Just search for the item on eBay and compare prices, and list it at what you believe your stamp or cover is worth based on comparison to similar items.
But really, it's not so simple. There is more to it than list it and sell it. Sometimes (i.e., frequently) a stamp is so common that it does not sell. So it is to your advantage that your stamp has more going for it than the average. Superior centering and pristine condition (no tears, gum stains, missing perforations; and it goes without saying, never hinged) will attract collectors looking to add a quality item to their collections. For an item that is clearly better than 90 % of similar items offered, go with a higher starting price. If using Buy It Now (BIN), make the sale price catalog value or above. If your stamp is near perfect you will get your price.
The False Promise of Graded Stamps
A recent trend is graded stamps; those that are graded highly and "slabbed" -- sealed in a plastic case -- have been known to fetch prices many multiples of catalog value. Such grading first came to collectibles in the field of baseball cards, where companies like PSA and Beckett graded cards that collectors sent in, hoping for a high grade and a good payday. The bloom is off the rose of grading, both in baseball cards and stamps. The poor economy can be partially blamed, although in the world of philately graded stamps never got a foothold with collectors the way baseball cards did with their collectors.
Finding and Bidding on Stamps on eBay
You must use many methods for searching for stamps on eBay. The most obvious is to enter country and catalog number. If you're bidding on U.S. stamps it is best to use the Scott catalog numbers; if British area stamps, use the Stanley Gibbons numbering system; if France, use Yvert catalog; if Germany, Michel. It is true that on the U.S. eBay site most worldwide stamps are listed by Scott catalog numbers. But don't miss out on those listed by specialty catalog numbers. A good philatelic literature dealer will have the catalogs you need. You can also find catalogs at stamp supply dealers, like Subway.
You'll want to survey the stamp dealers on eBay, using their feedback score as a recommendation. Soon enough you'll find one that features the items you collect, in the condition you desire. As you are aware of condition in your buying, it should be one of your primary concerns in selling.
Selling Stamps on eBay 101
So, while you know that as in selling real estate your top three considerations are 1) location 2) location and 3) location; the same thing applies to condition in stamp selling. Describe your item carefully -- misdescribing is the easiest way to lose points when your buyers leave detailed feedback.
If you're selling stamps of a greater value (your price point is up to you; many prefer $100) you must have the stamp expertised as genuine by an expert. After all you don't want an unhappy customer to return the stamp to you for a refund. And you certainly don't want the negative feedback that would accompany that return.
Getting the Stamp to Your Customer
Never use tape anywhere near the stamp when you pack the item to ship. Packing tape on the outside of the package is fine. Any sort of tape near a philatelic item is courting disaster. For stamps use a poster board thickness stiffener around the stamps that you have already put into a clean approval card, which you've put into a plastic cover or self-sealing plastic bag. You can put tape on the outside of the stiffener, to tighten the package and keep the stamps in place during shipping.
Many only use insurance for higher priced items - over $50 - though it's usually a good idea to use eBay's shipping service, which includes a USPS tracking number. Shipping stamps by UPS, unless it is a large lot, is cost-prohibitive.
I have only touched on the basics of selling on eBay, but it is enough to get the collector, dreaming of becoming a dealer, into the swim of things. Ebay has tutorials and many help and information pages which any prospective stamp seller would do well to peruse.