It really depends on which item I am searching for. I don't have one that's 1:1 replacement to Amazon, just at least few websites I go on for whatever I'm looking for.

• Clothing - Often use Vinted which sells second-hand clothes at a cheap price. Not great search filtering it has on both their website and their mobile app but you can find some decent stuff on there.
• Electronic/Gaming - I usually go on to eBay most of the time but sometimes, I would go on CeX and compare the prices. Often, CeX is overpriced but there's couple time it has been cheaper to get.
• Decoration/Gifts - Etsy has some nice items and stickers. It has gone downhill with the AI-Generated slop and has some dropshippers but there are some small businesses that make their own stuff which is what I love about Etsy. It feels more personalised and found some the fan-items to be better than some the official merch.
• Music - MusicMagpie sells second-hand CDs really cheap for anyone collecting CDs or just love buying that format. They also sells DVDs which I'm more a Blu-ray fanboy but I do also like my DVDs. I sometimes buy.flac music from Qobuz but I find them to be overpriced for me but if it support an artist, get the money I guess. Seem as I'm a broke-ass, MusicMagpie would do as I love collecting CDs and holding it sometimes just to look at the album cover.