It is my opinion that there is something fundamentally wrong with iPod Shuffles, including the latest generation.
My Touch shows up every time I connect it and so do all other USB devices. For reasons which escape me, my iPad always shows even if it's not connected.
I have had three 2G shuffles and three 4G shuffles and they all can only connect sporadically. Recently, I got a replacement 4G from the Apple store after they too couldn't connect my previous 4G to their Macs.
I also have a Mini Mac and my shuffles rarely connect to it. Yes I have tried the Apple troubleshooting checklist a zillion times.
p.s. don't get me going on why the Touch, the 2G and the 4G all require different USB cables.
Since you purchased these older shuffle's used, did you get the original USB docking cable that came with the 2nd gen shuffle (it has a mini-dock at the shuffle end)? The USB cable for the 3rd and 4th gen shuffle does not work; the connector is different.
This is the official Apple part
There are third-party replacements; just make sure it is described as being for the 2nd gen shuffle.