That's the metric I usually use. It's absolutely inconclusive, but it works for peace of mind at least. Have I seen any ads for something I bought? Usually the answer is "no". I'm still at the mercy of sort order on sites like Amazon and eBay, but that's much less scary because if I really care, I can sort by lowest price first.

By my own moral compass. If a company says "lots of people on Amazon are looking to buy what we have, let's make sure we're present in that market", that's completely moral. Compare that with, "a child is preparing for their math test tomorrow by watching a video, let's interrupt them and make them watch a commercial about our sugary, addictive, and unhealthy drink".

Is it even possible to not spend money on advertised things?

Mostly by not spending money frivolously.

The house I live in was advertised when it was on sale, all the food at the store gets advertised, all the non-bespoke clothes I can buy get advertised, as do most of the bespoke ones, every car gets advertised...

The difference is whether those ads were exposed to you before you bought it. Was it a factor in your decision?

Not whether it's something in the world that's ever been marketed at all, because literally everything has.

Generally yes. Especially the grocery store is constantly advertising hundreds of items, and I've looked at their ad pages more than once. And it's pretty hard to find houses outside of advertising platforms, and I used to watch enough TV that I'd seen ads for every clothing store and every car before I had a chance to find them on my own.