So, I bought some used OEM Sachs Sport Shocks that were in "great condition," according to the seller.

Get them, and one of them is like this...

http://youtube.com/watch?v=Sr2sopduI8E

Am I missing something or is this shock blown? Once compressed, it doesn't rebound or anything...?

If there is considerable resistance when you pull the shock in and push it back in, its fine. A shock/strut is solely a damper, as the spring/car weight is what return the damper to its original position.

it looks like it needs to be charged

a blown shock is when you can move that thing with no resistance at all, yours is fine, like tealetm said, all you need from the shock is to dampen the ride so you are not bouncing around on just a spring.

Ok, sounds good. Thanks for the info.