Ali Express = Straight from the Chinese factory town to to you. There are various things to watch out for. Like with electronics components the shops sell the stuff that didn't pass quality assurance testing for whomever ordered a manufacturing run, and now the factory is selling the rejects via AE shops.

Temu = Ali Express but now with more slavery

EBay = Anyone can do eCommerce but reputation carries enough weight that at least things usually aren't scams. A lot of it (like many amazon sellers in the US) are just middlemen for AliExpress, Temu, or similar but hopefully they get sniffed out eventually. It's like it's not the wild west any more because the railroad came through, but the laws between sheriffs are still real different.

Temu uses actual, honest to God, slave work

https://youtu.be/qnu_UcaRZv8

I've spent at least $5000 on aliexpress over the past few years, maybe more. I've never had an issue that wasn't resolved with a refund or a replacement. They're pretty reliable and I'd recommend them to anyone. They stock a wide variety of components and parts if you're building electronics or just general DIY / 3d printing / home stuff.

Temu feels way more sketchy. They try to shove more stuff down your throat, and they even have little mini games where you can try to earn free things. They're way more about just pushing cheap shit. I tried them once, felt dirty and have never gone back.

Temu is the wish.com version of wish.com.

They both just resell the same stuff you can find on aliexpress, but aliexpress has a lot more sellers.

I shop eBay for legit parts, used stuff, and tires.
The other two are trash.

Depending on what I'm looking for, eBay has been great for finding used CDs, games, and electronics for a lot cheaper than anywhere else. As for the other 2, you'll just get cheap quality junk that might even be harmful (as with the case of the Australian child who got seriously harmed by a temu product).

I've never orderes from AliExpress, but I considered buying from Temu a few times. Every time something popped up in an add that looked good it told me I had to download the app first, and then app kept trying to get me to buy other things instead of what I came there for. This is why now I refuse to buy from any website that makes me download an app to even see what they have for sale.

I've never had any issues with eBay, and I've used it a bunch.

My understanding is that AliExpress was originally aimed at B2B (business-to-business) transactions. So it kind of competed more with the traditional Thomas Register -- connect a business that wants to find a supplier, though in this case it took a cut on each transaction, sort of a super-distributor. But it seems to have shifted to have more of a consumer focus. Certainly the few times I've taken a glance, there are pretty clearly plenty of consumer-oriented things on there today. AliExpress is China-based.

I haven't ever done more than very briefly glance at the Temu website, but from my recent reading, it and Shein -- which you didn't mention, not sure if it's available in Australia -- they're a B2C (business-to-consumer) thing, more like Amazon. They're aimed at the value segment. My understanding is that one major factor that contributed to Temu and Shein doing well in the US was that low-value shipments from China to the US didn't have to pay tariffs. I'd guess that this was to help reduce the transaction cost of international sales, since any kind of red tape is going to be magnified if you have to do it many times over. So a vendor in China directly selling a pair of shoes to someone in the US didn't have to pay a tariff. Larger-value shipments, like a bulk import of a shipping container full of shoes, did. That meant that traditional importers, who would buy a bulk shipment abroad, import it, and then have it sold split up via domestic vendors, were at a disadvantage. I don't know whether similar factors apply to Australian customs policy. These two are China-based.

Ebay is US-based, was originally an auction site that targeted secondhand stuff. You can get new stuff there now, and not just at auction (and Amazon now sells secondhand stuff, albeit only at fixed, non-auction prices, so I'd guess that they compete more-directly with each other). I've only used it when looking for exotic expensive stuff that one can acquire cheaply secondhand, or stuff that can't be found new today.

Temu is like Wish but turned into an addictive gambling game targeted at children. (IDK if targeted at children is the right word but that's the vibe it gives me)

Temu looks SUPER sketchy to me. All of those "down load our app!" and "Just one more for 70% off!" deals and so on. I've never been to Temu, found an item, and checked out.

AliExpress and Ebay actually let you BUY things.