Having just bought a pair of upper arms for the 166 from Alfaworkshop, I too took a moment to ponder these cheap arms that are popping up on eBay. I didn't ponder for long though.
There's always going to be a limit to how cheap you can make something. If good arms are typically going for £50-£60, then I think it's reasonably safe to assume that the only way someone is going to produce them cheaper is so cut back on quality. The cheap eBay arms probably compromise in quality of balljoint/and or bushes.
Now, given that replacing these parts isn't the easiest of jobs, and each time you should really get the wheel alignment re-done, I wouldn't want to risk fitting something that could be knackered in a few thousand miles for the sake of saving £20.