The ring-flashes (on e-Bay or otherwise) have a trait that is not the best for certain applications, they leave a round, doughnut looking reflection in glassy-looking eyes. I used to have a Canon ring-flash, I sold it because of that, I mean, it's a VERY good flash, for what it does, but the ring reflection made me not like it so well. If you use it on bugs, like I did, it will leave a very unattractive ring-look in their eyes, well, bugs that have a shiny, reflective eye, it will. Jumping spiders, for instance have a very glassy-looking eye, and the ring-flash ruins it, at least for me, jumping spiders are one of my favorites. On the other hand, grasshoppers, and bugs that don't have a reflective eye, it works perfect!
As far as buying one on e-Bay, I have read here and elsewhere that some had good and bad happen with them. For what they cost, I guess it would be something that a person wanted to chance, but it would be just that, a chance it'd be what you want.
I read an article once that said to go to e-Bay and buy the older manual Canon ring-flash, I think they were made for 35mm's, they are just as good at producing what you need, but are really, really cheap to buy. Sorry I don't remember the article's website, but I wish I had known before I bought my ring-flash. (From what I remember, it was a safe voltage too, which is VERY important)
Personally, now, I just use available light, (with a reflective piece of white cardboard) I like that look more, so that saved me money. (Not really, 'cause I had already bought a 430II flash to replace the ring-flash, heheh)
In the case that you already have a 580, I'd definitely go with that, read up on the ways to get that to work for you, I seriously doubt the weight different will be that noticeable between a 580 and a ring-flash.