I have seen too many people paying too much for a coin that is valued far less than their paying price.  Here are some tips on buying slabbed coins (i.e. graded coins) from two major third party grading companies, NGC and PCGS.  I have been buying and selling either NGC or PCGS graded coins on Ebay for many years.  One advantage of buying NGC or PCGS graded coins is that their website contains pricing guide.  You can lookup the price of a particular coin by it's certification number.  The sellers on the Ebay either have "buy it now" price or just let the bidders take care of the price.   I stongely recommend looking up the coin of your desire by the certification number and see what is the value of the coin is.  If the price of the coin is near or at the catalog value, don't buy it.  For example, I have a 2004 Platinum Eagle $100 NGC PF-70 that is valued at $7500, according to the website, that is.  But I strongely doubt it that anybody would pay $7500 to buy that coin from me.  If you have been following 2004 $100 platinum coin at PF-70, it may go for around $3000 to $4000 on Ebay.  I have noticed that the going price on the Ebay is about 30 to 40 percent lower than what is stated on their websites.  So, before you start moving that mouse to bid or click on a "buy it now" icon, do some research.  Doing  a bit of research on the price may save you some money.