What's the last major thing you purchased? Basically, be on the lookout for frustrating purchases, were you not able to find good info? Too many spammy sites? What site do you wish was available to help you with that purchase? If you can't find it, make it.

I can only think of price-comparison or review-based websites. Is that what you're talking about? How could you monetize something like that without ads, for instance?

curated lead generation. You basically only market the good products in your niche

They're making their living off it, won't likely tell you. I can understand that.