Hi, everyone!

I was cleaning my bike, and notices that the rear derailleur top pulley is skipping the chain. Checked the chain, and it was good.
So, I decided to take off the pulley. Opened the cover, and.... dropped on the floor. The balls are tiny as molecules, I was unable to find them. I am using Tiagra pulley on my Dura-Ace RD temporarily and chain skipping stopped. But I would like to replace RD pulley

I have few questions to bike mechanics experts:
Do I have to replace RD pulleys regularly and if so, how often?

Do I have to replace the pulley, or just the bearings?

Does it matter what pulley goes on top and what on the bottom?

If so, are the cheap eBay parts will work ok?

Like this one:
http://www.ebay.com/itm/New-11T-CNC-...item4174f02c0f

Thanks!!!