Depends where you are in the world, and I am less familiar with the Italian flavours of both than I am many of the others (I know US, UK, France and Germany for Amazon and their warehouses and policies, they can be quite different as well -- people talk about prime in the US giving free shipping, it is and usually was always free in the UK, speaking to some of the Dutch people then they can try to have addresses in Germany as the distance can be longer across it than it is to get to the Netherlands but shipping can still shoot up and vendors not want to ship things).
Ebay.
You find a lot more random stuff. I am not going to find car parts on amazon, give or take electrical boring stuff and maybe the odd gasket set, ebay on the other hand. If you want old and cool things, that are not books and games anyway, then amazon does not even compete.
You also find a lot more "broken" stuff. I can buy spares and repairs things for all manner of devices on ebay, less so on Amazon.
I see some bigger "bundles" on ebay, and at the same time people parting those bundles. Worth having a look on both if you are after job lots of electrical parts though.
Amazon UK at least does better for a lot of newer tools and machinery, at least outside the pro market. On new gear then I see enough on both of them, though Amazon probably has the edge in consumer goods.
Amazon do better for games and books for me, if I am after specifics. If I am just going to browse randomly and see what there is then I will go in real world shops but if I did have to use online then ebay would be part of it. Given ebay is the very thing that kind of put an end to the "I will sell you this rare book/game/tool/... because I think it is old junk" in second hand shops, newspapers, jumble sales.... then there are a surprising amount of people on ebay that do not check ebay prices.
For most electrical things they are pretty even on basic "new" replacement parts. If you want replacement laptop screens, tablet screens, charge ports, antennae... then there is not a lot in it. In any case if the seller does it properly and looks like they have a normal website (my chosen vendor for screens is on amazon and ebay, they have a site with everything usually £5 cheaper for the same parts and everything, and I have similar stories for flash memory) then go that.
Consider that you also have things like craigslist in the US, gumtree in the UK and a quick search told me
http://www.subito.it/ in Italy.
Amazon stuff sold by Amazon is like a normal website, however everything else with random businesses and members of the public I consider to be the bottom rung of selling stuff online.
Seller ratings on both went kind of meaningless years ago -- always love the little notes that say contact us before you give us less than 5 stars thing, and given that sellers can rate buyers and be vindictive... Though the usual check the language and if it looks like it was written by a 13 year old that can not write, and also does not know the first thing about what they got, then consider it accordingly.