Ebay is not bad, generally speaking. One needs to look at seller's feedback, and how many items have been sold. I've been selling and (Mostly) buying on there for 17 years, and I'm a premier seller. I've gotten burned a couple times, but nothing major and it was early on while I was still learning the ropes of it. Some great deals are there, and it's most likely safer than Craigslist (which I also buy from occasionally) since you don't meet the person somewhere. @Guardian_Angel or @airoutlaw if you ever want to get into Ebay, I'd be happy to help you with what I know about it.
As for the swing arm being solid or steel. I would rather it was steel myself. In this case, with what it is designed to do, I think a tube steel design could be not only stronger, but not add much weight if any. That would be my first and best choice.
I believe Bullet Speed has one that is tube and works well. Since its not mass produced though it is a little pricey.