It's good to have friends who hunt or shoot. Ben Franklin has bought me 3 guns so far: Mossberg 500 (with smoothbore & rifled barrels), Remington 870 and a S&W .357 revolver. 3 guns for $300! That's still less than what I paid for my S&W .38 snubbie! Even a reliable friend of a friend might even have something for sale. As long as they can be trusted and you fill out the right paperwork, you could get a good deal.
Gun dealers always seem to ask me "Is it for target shooting, hunting or home defense?" No matter what my answer is, their response invariably is "Then you don't want to rely on a used gun." You absolutely do not need a new shotgun for home defense. But you need to inspect & clean it first. If you're not qualified, take it to a gunsmith. It's worth the investment. I never rely on a used gun until it's been inspected and cleaned and I never rely on a new gun until it's had MANY rounds put through it. My 870 is new to me and while she's a 1950's model with surface wear, she functions smoothly, shoots straight and hasn't misfired, failed to eject or failed to load. This was after running 150 skeet rounds through it plus about 35 rounds of buckshot.
You could stop here if you want and you'll have yourself a reliable HD shotgun for about a hundred dollars. Remember, you're defending your home against intrusion and not playing a videogame. Five rounds in the mag should suffice. I plan on pimping mine out by chopping the barrel, refinishing it, adding COPstock folder, mag extension, sights and a light. I don't need to do it, but this is my latest gunsmithing project and it keeps me out of trouble - sort of... not really...
-G