I don't use any forums, but I buy a LOT on AliExpress. Like, really a LOT. Staggeringly a lot. I'm a Diamond level user there.
I also use Amazon for some things and eBay occasionally. Banggood, sometimes, but not much.
I buy mostly electronics stuff.
Oh, I had 1 GREAT STEAL on Amazon where I bought a 65" 8K TV to use as a computer monitor. Snagged that puppy at less than half price!
However, here's a great strategy that works on them all...
1. Find what you want, but don't buy it - have patience
2. Search around for it more
3. Click on the suggestions that are sufficiently similar
4. Keep clicking on those suggestions
5. Add things to your cart, but don't buy. Keep them for a few days or whatever - you can delete them after some time. Think of it as "saving" things - forget the wish list.
If you do that for a while, the platform that you're using will start to "understand" you and serve up better results for you. With AliExpress and Amazon both (probably the best 2), you'll keep getting the same recommendations, but new ones will creep in. You'll eventually get what you want at a great price.
Also, things to watch carefully:
1. Seller reputation
2. Number of sales of that item from that seller
3. Customer reviews of the item
4. Shipping costs
Make sure to read reviews.
For AliExpress, often things are so cheap that you're likely best off buying the same thing from 2 DIFFERENT sellers. Some ship much faster than others. Also, you may not get what you want from 1 due to customs or some other problem, so having 2 orders is often useful. (Could say the same about Amazon as they can suck just as bad.)
Another thing to look for is official stores for a particular brand/item. You can often get them cheaper and more reliably then from other "no-name" sellers.
Make sure to compare AliExpress vs Amazon as well, because for larger items or some particularly common items, you may get a cheaper price on Amazon and need to wait less.
It's also worth checking against other brick & mortar retailers, e.g. Walmart. I picked up some styptic liquid that goes for around $22.30 CAD per bottle, but I got it from Walmart for $57 or so for a pack of 12. WAAAAYYY better prices there.
For higher end items, it's often best to just bite the bullet and order from the most reliable place possible, even if you need to pay a few extra dollars (or euros or other fiat shitcoins).
Anyways, not about forums there - just me blathering about what I've seen.
In most cases it seems like online communities related to a particular product are usually the best resources.
Bang on the money. Search forums dedicated to a particular product category. or YouTube, or whatever. Facebook groups or anything.
Make sure to also check out my most recent thought crimes... Mwahahahhahhah!
I've been naughty, and it will only get worse!