I thought I'd pass it along some thrift store tips that have worked for me.
1. JUST MISSED IT
Say you're at a thrift shop and someone has an awesome game in their cart. Don't be afraid to go up to them and say, "Wow, that's a wonderful game. You're lucky to have found it." Sometimes they will say, "Oh, I don't know anything about it. I was just carrying it around and will probably put it back. Do you want it?" YES!
Also, after they have left the store, you can politely say, "I noticed you bought such-and-such game. Is there any way you would be willing to sell it? I will pay you twice what you just paid (note: or whatever amount you think it's worth)." Caution: do not do this IN the store! Most thrift stores frown on this.
Some people will be very happy to make a profit right away. Maybe they were going to list it on Ebay and you have saved them the trouble and listing fee and trip to the post office. Maybe they just thought it was a cheap game to give to their grandkids and would rather have the money.
2. SAVE IT FOR ME
If you a regular buyer and are friendly to the employees, next time you are at a register, you might want to say, "I'm always on the lookout for board games." It doesn't hurt, and they might have something for you next time you're there.
3. MISSING PIECES
If you check out a game and it is missing pieces, the first thing you should do is look all around the area to see if they fell out. Under shelves, and even inside other games. I have found several cards to games this way. Second is you should decide whether it is worth getting even incomplete. There is a good market on Ebay for game parts and good trade possibilities here on BGG. Also, you MIGHT also be able to get a little bit off the price with a sympathetic thrift store employee, but don't hold your breath!
Never leave behind a good game just because it's missing dice or a timer or generic markers. You can either easily get replacements yourself, or sell/trade saying it needs these parts and many gamers won't care as they have plenty of dice, timers,etc.
4. BAD CONDITION
There are several discussions here on BGG on how to rid games of mildew and smoke (do a keyword search in Forums). If a game is cheap and potentially valuable enough, it's worth trying out these methods. In the case of games that smell like smoke, even if you can't get the smell removed, you might still be able to sell/trade the game to a smoker. There must be gamers who will be happy to have at least one game they can play while they are smoking, or who want to bring a game to a pub/bar.
King of the Dead
All the thrift stores around here put about half a roll of packing tape on the boxes so you don't know what's inside until after you buy it.