Sure, we have a wiki of various price comparison websites, but what it doesn't say (currently) is how to use them to our best advantage. E.g.:
What are the best ways to search on eBay, or Amazon, or AliExpress…?
I find eBay has the best search engine, given its plethora of filters and ability to use Boolean searches (hyphen for NOT, comma-delimited list in parentheses for OR). It's also the best for sorting, as it includes delivery costs in the sort. Amazon is second best (in terms of searching); it doesn't have as many filters, doesn't do Boolean searches, and the sorting doesn't include delivery. I find AliExpress a nightmare to search (@Clear is probably more proficient at it, and has some tricks up his sleeve). To get around its shortcomings — no filters whatsoever, sorting all over the place — I generally either sort by price filtering for free delivery, or I sort for best sellers to filter out duds and fakes.
Which site is best for searching for each class of product?
I find AliExpress is best for no-brand light-weight tools, toys (knock-off LEGOs), and random knick-knacks generally sold in dollar stores and Kmart. eBay is best for local stock of bulky, heavy stuff, office supplies — inkjet cartridges, print transfer sheets, etc — and for some brand stores offering better prices there than anywhere else. I don't use Amazon much, other than to read the reviews on branded appliances. Almost every time I compared it to iHerb on supplements, I ended up buying on iHerb instead. I also reckon iHerb has greater variety, and more informed reviews than Amazon.
If this thread amounts to much, perhaps we can turn it into a wiki for future reference.
I use ozbargain.com.au