If you are not an experienced coin collector then the appeal of coins that are encased in a plastic holder that certifies them as genuine and correctly graded as to condition is appealing.   Oh boy! 

Watch out! 

Why?  Coins that have been handled this way are referred to as "slabs", refering to the plastic case in which the coin is encased.  The recognized quality certification and grading services charge a fee for their services.  Once the grading service has been completed, they put the coin in the protective holder to ensure that their evaluation resides with the proper coin.  Additionally, the "slab" protects the coin from any deterioration.  There are three (3) widely reputable, accepted, third-party grading and authenication services - PCGS, ANACS, and NGC.  These firms very rarely make mistakes regarding authenication or grading.  Not infalliable, just rarely wrong.  I also like ICG as well as PCI.

But, since the term "slab" has come to some prominence, individuals and small firms have taken to the fray and added their respective personal, self-serving slabs.  Thus they have become known as self-slabbers.  And their opinion regarding authentication and condition is entirely, completely, withoug question, wholly self-serving. I try not to be wishy-washy on that point.   Most put them in their own labeled slabs and sell them under a different company name either on eBay or elsewhere to inexperienced collectors.  Then, one day the collector takes the coin to a dealer or to a coin show and learns the truth that the coin may be worth a lot less, ohhhh sometimes a whole lot less,  than what they paid for it.

So, what to do? 

Well, you can learn grading and evaluation of coins, but that is not always foolproof since internet photos can be manipulated to show only the best.  Grading on the internet is not for the inexperienced and sometimes even the experienced ones get fooled - Tuesday Night Coins and Vic Brenner are two dealers that come to mine that do not truly represent what they sell.

Or, you can acquire coins that have been graded AND authenticated by PCGS, ANACS, or NGC.   Personally, I also like PCI and SEGS but they do not have the universal reputation that the top three do.  Using these services and avoiding all the others should help you avoid being taken. 

Yes  you might miss a bargain, but there will be other bargains, AND an overgraded or counterfeit coin is never a bargain.