Loose games, whether CD based or on cartridge, are always worth less than complete ones. Manuals and boxes will always bring the value up, and generally, the price depending on how knowledgeable the seller is. There are many specifics here, such as games that were released without boxes or manuals. Some games were sold with special items, like maps or hint books. In the case of Earthbound on the Super Nintendo, the scratch and sniff stickers could bring more than the game itself if they're included.
The same goes for the consoles to play all of your new games on. Ensure all the cables are included, including controllers. If possible, check the console yourself by hooking it up before purchasing. If not, simply ask when the last time was the system was used. These things rarely become defective unless extremely abused, but there's no reason to take an unnecessary risk. Have some sort of assurance.
Condition
There is no grading system for video games. Mint is still mint of course, and whether or not this matters to you is a personal preference. Cartridges can take a beating and still work, so if you only want to play these games, then one that's been drug through the mud won't be a problem. Cleaning them is easy, requiring little more than a q-tip, alcohol, water, and some time. Just rub a mixture of 50% water and 50% alcohol on the cartridges contacts and dry it off with a clean towel (though not paper towels; they can tear and stick to the contacts).
Disc based games do have problems if they're damaged, and only experience can tell you if a CD will work properly or not with heavy scratches, and even then it's an educated guess. If there's a chance in your mind that it won't play and the price it high, just pass. Most stores sell disc repair systems, and their success is spotty at best. Even a full resurfacing, generally a service available in specialty shops, doesn't always work. It's best to play cautious.