First of all as a game collector you have several options where to look for your stuff.
I suggest doing a little research before buying anything. Unless you get a box of stuff at a yard sale for $5
Say you are feeling a little nostalgic for some pacman. You decide you want to get a 2600. What now? In this wonderful age of information it is now possible to buy a complete Atari 2600 setup with 50 games and have them delivered right to your door. The trick is to get the best deal. Unless you live in an area with a lot of flea markets you're not going to find this at your local yard sale. Ten years ago maybe.
People complain about eBay. Sellers complain about not getting enough and paying high fees. Yet buyers complain the prices are too high. eBay is a wonderful way to gauge the value of a video game. And if you want that 2600 with the 50 games there is an auction ending in a day or two with a $37 dollar bid right now :) And don't forget the $10 shipping and handling charge the seller has tacked on. But if you use your search skills you might be able to find someone selling a 2600 with 25 games for $25 plus UPS shipping. I know guys that sell Atari games for $1 each. Even the guy at Zern's farmers market with the old games only charges $2 each. That's better then paying $4 bucks for a Frogger on eBay :) It amazes me that 5 years ago some of the rare Atari cartridges like Pigs in Space were going for $15 and now you can get them for $6.
There is a local GameWorld store that has about 10 old Atari cartridges listing for $2.99 each. Combat and Space invaders and even one I actually bought. The Challenge of Nexar. A spectravision cart that would have been worth $15 five years ago. Now the last one on eBay sold for $2.99. Supply and demand. Low supply, but low demand. And a Chase The Chuckwagon game still goes for almost $200 on eBay because they are rare. But there are people out there listing theirs for $200 for sale anyway.
You might find a Sega or Nintendo at a local yard sale now. Unfortunately most people that have a Sega also have 7 or 8 sports games with it and they want $25 dollars. Most of the Nintendo people part with them for $5. $10 for a Super Nintendo. Here is where you really need to do some research to avoid paying way more than you have too.
There are some Sega genesis games that I would have no problem paying $10 for. Because they are good and worth it in the long run. There are even a few games I would pay $20 for if I didn't have. It is my personal opinion that a Sega is the best value right now to get a system and a few dozen games to keep kids 5 - 9 occupied. My 4 and 5 year olds play their Sega almost every day. Finding a Busytown for $9.99 or a Mickey Mouse circus mystery for $5.99 at the local Gamestop is a good thing. But they end up playing Sonic. All the time. Mostly Sonic 2 since they can both play at the same time. And for variety they backpack it on to Sonic and Knuckles.