Actually, I'd suggest you buy a new freewheel through your LBS. MF series freewheels for 6 speed are still available and they only cost $7.50. 7 speed is $8.50. That's going to be cheaper than any Ebay purchase once you factor in shipping, plus you know it's new and have a warranty.

However, while you're at it, you might just as well pay the extra $5.00 and get a Hyperglide compatible freewheel. They have profiled teeth that allow you to shift under heavy load. Of course, you'll need a Hyperglide compatible chain, but if you're replacing the freewheel , you should replace the chain too. Personally, given how cheap HG freewheels are, I'm suprised that anybody would even consider riding an older style, unless they're fanatics about having their bicycles 100% original.