I am a collector who has been buying and selling raw coins on eBay in his spare time for the last 7 years. Many collectors and dealers tell me that buying raw coins on eBay is a waste of time and money. Nevertheless, this is how I've built the majority of my collection. I will be somewhat surprised to see if eBay publishes this guide. Just in case, here are my experiences.
Raw coins on eBay need not be problem coins. Certified coins don't make economic sense to exist unless they are worth a certain amount. Why would the submitter pay registered mail postage both ways plus a $10 to $30 per coin grading fee for a coin that sells for less than $50? I have built a nice collection of Fine to XF Barber coins without the grading services guarantees and without enriching the USPS or professional grading services. I have learned much about grading and problem coins by reading coin grading services' online forums.
If you are buying raw coins on eBay, remember that at least 90% of what you see is not for you. Look for auctions with large, clear pictures and learn how to tell from a picture whether a coin has been cleaned. Ebay requires coin sellers to fully disclose a coin's problems and grade it to ANA standards, but very few sellers do this, and the ANA does not spend any money hiring experts to police eBay auctions of problem coins. So it's "buyer beware". How can one tell a good seller of raw coins from a bad seller? For starters, beware of private auctions, auctions with "estate" in the title, and "certified" coins by a seller whose slabs bear the Coin World logo or who is one of very few sellers of coins slabbed by a particular service. There are many acronynms for small grading services which hawk their wears on eBay, hoping for a price better than what they would get if they sold the same coin raw.. If the coin is not PCGS, NGC, ANACS, SEGS, or ICG, look carefully at the picture and description before looking at any price guide. Self-slabbers posting on eBay are merely looking for a wide audience, which might include a newbie thinking that all "certified" coins are graded equally. If the coin is raw, think at least twice about bidding on a coin with no return privilege. Ebay has become a dumping grounds for a dealer's inventory which would not sell to other dealers at a major coin show. Much of what you see here from professional dealers (or advanced collectors) are the problem coins or low grade coins which would otherwise sit in their inventory for a long time.
A seller's feedback and PowerSeller status is often meaningless. Unless a coin seller's feedback rating is below 99%, you should suspect that this person stretches the truth or is lazy. Most eBay coin buyers don't sell coins often enough to realize how badly they've been ripped off, and they post positive feedback for prompt shipment on an ugly coin the day it arrives in their mailbox. Some post positive feedback for fear of a retaliatory negative. My own experience on eBay has 3 negatives in response to 3 raw coins I was stupid enough to buy from dealers despite red flags in the auction description or pictures.
There is no Santa Claus in numismatics, but there are occasionally good raw coins to be found on eBay. Some of my best buys on eBay were on holiday weekends with few bidders. Most of my eBay purchases were, in Fats Waller's words, "slightly less than wonderful". Other good purchases on eBay were auctions without the usual keywords in the title or those listed in the wrong category, For normal bread and butter coins, I still look to eBay, but search on auctions from a few trusted sellers (mostly collectors) whose images I can decipher.
Still, eBay provides a much better selection of coins than any local coin shop, and provides a useful service matching collectors of similar items. I have made many friends buying and selling on eBay and have made a good "favorite seller" list from my experiences buying raw coins here.
Ebay is still a better way to trade coins than the usual retail coin shop. Ebay and PayPal's take is usually 10 - 12% here vs. 25% at a dealer or major auction house. Ignore most of the auctions here, concentrate on a few, and you will do well