17" G4 iMac Purchase: Yay/Nay?

70 dollars is a good price if it's in good condition. You'll need to see how bad the monitor arm is, if it bothers you or not and if it's maybe fixable?

I bought a 20" iMac G4 (1.25 GHz, 768MB RAM which I later upgraded to 2GB RAM) on eBay in January. In the UK this kind of model still goes for about 200-300 pounds (!)

you really need to see the neck first to make a judgement if its a easy fix or if it needs a rebuild
usually they get lose a bit and just drop down or only stay in one place ,
which would only need the screws (special screwheads with 3 holes )tightening again , but you have to open the neck to look inside first to make sure everything is how it should be inside ,
if the neck is bend , it would indicate that the iMac was dropped at some point in life too
and there is a spring inside which can become a bit dodgy too if someone (kids) think its a bouncy castle feature

but for the price the iMac g4 is on offer its well worth even if you would need to replace the whole neck if you cant easy fix it or live with it like it is

here a take apart guide in pictures
http://homepage.mac.com/mactechg4/PhotoAlbum22.html

I purchased a 17" 1GHz USB 1.1 model for $125 about 2 years ago. The one you're looking at is a much better machine, and for less cash. If the display still functions properly, and sits at an angle that works for you, then its a good buy.