There is an alphabet soup of third-party graded coins being offered on Ebay at any given time.  As in any hobby, the collector should educate themselves regarding their hobby. 

Rule #1 - Buy the coin, not the holder.  Don't assume that because the holder states that the coin is an MS70, that it is.  Don't be lazy, take the time and and learn how to grade.  An error of a single point on a rare coin could cost upwards in the thousands.  I can' tell you how many collectors I have encountered that have bought MS70 packaged coins only to find out that the coins were not worth much more than pocket change. 

Rule #2 - Investigate the third-party service.  Do they have a website?  Do they have a phone number?  What do their coins sell for on the open market?  Do they take submissions from collectors?  Do they guarantee the coin's grade and autheniticity? 

Many slabbed coins are purely coins in packaging by the owner of that particular company.  Ask coin professionals what grading services they will buy a coin sight unseen and the only ones that will EVER come to mind are PCGS, NGC, ANACS, and ICG.  Any other services do not carry a significant reputation to warrant the risk of purchasing an item sight unseen.

Rule #3 - While grading is subjective (a matter of opinion), there are generally accepted standards that have been in existence for many years.  Aquaint yourself with the American Numismatic Association's ecucational material about grading coins.  This is the standard of the industry.  Interpretations can differ.  A reputable dealer will accept a return on raw coins due to that fact.  Don't accept the statement from an ebay seller who states that they believe the a raw coin grades MS63, but refuses to accept the return of the coin because you don't agree.  As the buyer, professional numismatic organizations require that sellers of numismatic materials offer a return priveledge.  Anyone who would not accept a return is unethical in their dealings.

Rule #4 - Avoid dealers that represent values of their slabbed coins using the PCGS or NGC websites (Coin Universe and Numismedia) yet are not selling you PCGS and NGC coins.  These websites are designed to indicate the value of their respective holdered coins.  For someone to tell you that their own holdered coin, graded by themselves is worth what a PCGS coin is amounts to misrepresentation and fraud. 

Rule #5 - Ask questions, get answers, and ask more questions.  If the dealer does not want to invest their time in helping you learn about the hobby, move to the next dealer.  Find someone that you are comfortable with.

I welcome your input and desire to make Ebay a safe and effective place to buy and sell coins and other collectibles.