It depends on what you're buying - I don't buy (commodity) books much anymore so can't comment on those. What I can say is for "random tat" that I don't need quickly, Temu is an unabashedly "cheap chinese stuff" that is often the same as the Amazon version, but usually a lot cheaper. I'm assuming they charge less fees to the sellers. They deliver in under 2 weeks. AliExpress has now started offering the same service. Downside is there's not really returns - or at least I've never wanted to deal with the hassle. However, for most stuff under $10, I wasn't going to return to Amazon either.

For "Brand Name Stuff" I kind of go to ebay and/or the specific retailer like Best Buy or B&H or the manufacturers site. They seem far less likely now to have counterfits because with e-bay the actual seller is tied to the specific product where Amazon isn't, and the other stores don't want counterfeits and have a more controlled supply chain.

For stuff like spices or the like, ebay or Walmart.com seems reasonably good. Walmart also has a lot of random sellers, but as far as I can tell, they don't do the binning Amazon does, if you buy from Walmart it's from them, if you buy from a third party, it ships from that third party.