i'm pretty new at this so i really don't know what to look for here. i know to find out what casting number the heads are and that the bigger valves are good, but what else should i be looking for when buying used heads. i found these and they seem like a good price but i don't want to get ripped off. have a look:
http://saskatoon.kijiji.ca/c-cars-vehicles-parts-accessories-454-Cylinder-Heads-W0QQAdIdZ123846073

It's very difficult to evaluate them visually, anda dn you never really know what you;re buying. At a minimum you'd want to get them to a reputable shop and have them checked for cracks, pressure tested, and maybe decked ( a fresh surface for the gasket), then assembled with the valve springs that are correct for your cam. So you're still looking at maybe the same $350 to get them ready, and that assumes the guides are good, and the springs don't need replacing.

thanks, another newbie question here. what could happen if i just bolt them on with out any machine work. i actually found out these heads were rebuilt by a decent shop.

You still need to have them evaluated by another set of qualified eyes. What counts as 'good' at one shop may not pass muster at another. Find a local shop you trust, and take it from there. It wont' cost much just to have them inspected, and if they check out, you know you can bolt them right on with confidence.

And Yeah ,you still need to get the casting numbers.
Could be someone stuck large valves in a peanut port head as the pic doesn't show ports.

unless you have a stock cam it would be a big mistake to assume the valvesprings will work.

Have them cleaned and magnafluxed... (About $75-$100 total in my neck of the woods...