I’m not buying as many parts as I did “in my younger days”, but over the years I’ve frequently used eBay as a potential source. It pays to have a good network here to call on as well.  That dates back to before I started using eBay. 
Froggie

eBay parts have to be either cheap or not obtainable from MVA such as extractors or hammers (MVA hammers are of low wall height).  Springs, screws, flat spring and forend mounts are usually more expensive than MVA.  I recently won 2 triggers and 2 extractors for what 2 blank extractors would have cost me.  One of the triggers will free up a set trigger in my BUG (butt ugly gun) that needs some r&r but will see duty in my next set trigger build. 

Jack

It seems the best deals also come from auctions ending in the  middle of a week day. I always end mine on Sunday afternoon when buyers are usually home.   Those rare super buys are often buy it nows that are new listings.  I did get a good price on a Celestron spotting scope when the seller kept dropping the buy it now price every couple of hours. It started at over $450 and I jumped on it st $275.   
On the negative side I just received a barrel for a Stevens single shot 22 pistol that looked like it would fit my model 35 but is WAY smaller.  Back on eBay eventually.

I've found some very nice things on eBay, however, I don't get into bidding wars. If there's lots of interest in a particular item, part, etc., I let it go. From time to time, there are some really interesting things that show up.