I used those first examples as they are things I have needed  and they were not available at ANY retail shop anywhere and I drove all over Melbourne, or at least, to several places including one massive craft store. I wasn't just after a piece of ribbon, I was after a particular shade, in a range of particular, exact widths and I needed quite a bit of it. Ebay came up trumps.

I thought of the nigella seeds as I bought some on ebay recently and if you can find those in the 2 dollar shops I would be surprised. I couldn't find any in our local supermarkets and we have three. Ebay, easy to find and in just about any quantity.

Admittedly, that doesn't make any of them big purchases like collectables or electronics but I'll be honest with you, I would hesitate to buy electronics on ebay and probably on gumtree or FB either. 

As for collectables or books that are hard to get, I'd venture that ebay would be more likely to consistently carry some items you're after than either marketplace or gumtree.

I recently wanted a book called 'No privacy for writing' 1995 edition. It's not rare but it's not the sort of thing you're likely to come across in op shops either. There is 0% chance of finding it at any particular time on marketplace or gumtree but there are a couple of copies on ebay. And several on dedicated online book sites.

I agree with you it is horses for courses but the point I am making is that for most buyers, whether  they are looking for smaller items or a leather jacket or a collectable, they are more likely to consistently find a few of those items on ebay.

What the young ones use marketplace for a lot is to buy & sell second hand baby stuff, second hand children's clothes, shoes, toys etc and some furniture items. It's a bit like the original ebay, a hodge podge of private sellers. 

Probably should add the one area ebay wins out is that things are usually posted. An awful lot of people don't like face to face pick up, they like to be anonymous & this is particularly so with adult clothes etc & smaller items, though of course with furniture, different.