My best second hand purchase would without a doubt not be  a RPG product but game accessory.. my Dwarven Forge ! When I first started collecting Dwarven Forge I had planed to pick up one set ever few months and slowly build my collection.. It would take some time but it would be worth it..
Then I found a lucky auction on E-bay. I picked up a great auction for an extremely good deal. SO I took the initiative and inquired to him if he had any other auctions.. His response was fast, He messaged me and his reply was that he actually did indeed have sets. AND many of them. He was also in need of fast money. So after a few messages back and forth. We came to an agreement that he would ship the entire lot of sets to me for the low low price of 600 bucks..
600 bucks would usually pick up up four to five sets if you picked your sets carefully.  But the stack of Dwarven Forge that I got that day was Massive. If I recall properly it was

Two Oger’s Den’s, Octagonal Room Set, Diagonal Walls Set,

Curved walls set,  Deluxe Room Set, Wicked Additions 2

Caverns set x2.  Room & Passage Set & Wicked Additions

The results allowed me to provide my players with things like this !

Now go take a look at their awesome stuff here ! http://www.dwarvenforge.com/

Leave a comment below about what your best purchase was !