The Case for Certified Coins
Problem coins
But fake coins aren't the only reason to buy certified coins. What about problem coins? There's a popular adage, if it's not slabbed, you need to ask why. As you can see from the 1923 half penny example, good certified coins sell for more once slabbed, so why aren't all good coins slabbed? Well most are - uncertified coins of significant value are almost always cleaned or have some other fault that prevents them from being certified coins. This means the saving you make buying such a coin is typically all lost because the coin isn't worth the grade the seller gave it.
Consider this coin, a 1936 florin called UNC. If you look at the recent sales of certified coins below it may seem like good value with UNC-graded coins selling between $140 and $300 but this coin is actually a harshly cleaned AU. The buyer could've bought a certified problem-free AU58 for just $39.
Rule 2: If you find a high-grade coin uncertified, there's usually a reason why.