Sturmey Archer 3-Speed Hub Slipping? I'm about to go look at a 1972 Raleigh.  The seller says that the bike slips out of third gear into neutral.  Is this a simple fix (e.g. a cable adjustment), or should I stay away from such a purchase?

Also, when I go to inspect the bike, what should I look for to determine the difficulty of this repair?
Daniel summed up the first half of your question pretty well. For the second half, start by visiting this link to learn how to adjust the hub. When you go to inspect the bike try what you've learned first. If the shift lever is obviously sticky it may just need new cable and housing. Other than that, it's hard to tell. Internally geared hubs typically aren't a component that you visually inspect. If it's potentially trashed, 1972 isn't really a vintage that would be worth rebuilding. If the hub is trashed and you're really in love with the bike, you could always replace the rear wheel with a new one. 3 speed internally geared hubbed wheels are typically pretty cheap. Or you could wait for another bike like it to come around. Most bikes from around that time period are a dime a dozen.