Ich bin ja, Herr, in Deiner Macht
It really depends on the circumstances....I figured out a little too late that Amazon and Amazon and EBay are definitely not the first places to go to buy kits...
I don't buy kits, new or old, through Amazon at all. I'll buy new kits from an online retailer, like SprueBrothers or HLJ first. Or from the producer, if possible. For example, I've bought directly from CMK. Amazon is just the middleman, in any case.
As for eBay, apart from the DoraWings kit I mentioned above, I'm buying second-hand kits. And there, I'll look for the best bargain I can get, like a Buy-It-Now offering with a low asking price and low shipping. Like I said before, I've got ideas of the prices I'm willing to pay for specific items, and I'm patient, so I can wait till I see the bargain I want. It's kind of like fishing.
I don't buy any supplies at all through Amazon, or on eBay, either. I buy my supplies locally as much as possible. We're fortunate here in the Lehigh Valley in that we still have a couple of old-time local hobby shops-including the longest continually-operating shop in Pennsylvania, open since 1948. And the arts & craft stores are good places for some things, too, places like Michael's or HobbyLobby. We even have a local Dick Blick store. So I get my paints "fresh" from the store, as it were, tools, also, or I order those from the producer or other online retailers.
It takes time, you build up your own store of knowledge about where you can find things.
Oh, and I'll put in a plug for another online seller for supplies for figure painters: Dave Youngquist of Last Cavalry, website is http://lastcavalry.com/. Dave does the figure show circuit, as well as selling online. He also has a YouTube channel where he shares his tips for painting figures. I think he also collaborates with Michigan Toy Soldier on that effort. Dave's a good guy to get to know.
That's something else you can build up over time, too-relationships with vendors and other artists, and that network can help you find things. too.