Once I decide on a gun, say a police trade in Sig P226. I start looking on several sites that frequently have them, and then the auction sites. I don't care if the guns I'm looking at have some "handling marks" on them, as long as they are in good+ mechanical shape. If I see one at a good price, I will put check it out and buy it or put a bid on one. I don't bid on more than one of the same gun at a time, as I won two of them within 5 minutes of each other once, and I had to come up with twice as much as I intended to spend. If I don't see anything I want, I just keep checking, and soon, what I want for the price I want will show up and I'll win or buy it. Patience. I can wait a month, it's no big deal to me anymore, to save a chunk of cash, and I rarely have to settle for anything that isn't exactly what I want as to barrel length, color, etc. It only took me 4 days to find a decent P226 DAO for $350+ shipping. For about $60, I converted it to DA/SA for a total of $410. Not bad.
Patience and looking at EVERY listing is the key. Check out the pics, and ask questions if you aren't sure. Put in your highest reasonable bid and wait until the end, and see if you hit.