It seems to be cultural, and very weird for a secondhand shopaholic.
I've lived here twice, last time 20+ years ago for 7 years and every Weds night we used to drive around picking up all the furniture, white goods etc that had been put out for the rubbish men. (Leather Chesterfield 3 piece suite one night, and untold antique tables, dressers and so on) There were plenty of ex-pats looking for dishwashers, wardrobes etc - we often took orders and kept our eyes open. If the item was wrong, it went back on the street the following week - someone else might want it..... or lit the BBQ!
Recently, I picked up a perfectly good Hoover with attachments from next to the bin opposite. Which happens to be near the bar, so he may have just needed an excuse for a beer....! Another day we got a backpack strimmer on the way back from shopping. Both work fine. In UK, you would have put them on ebay or freecycle/freegle, wouldn't you?
Sometimes, maybe people would far rather be seen to be throwing it out and changing their decor, whether or not the Jones have or the economy permits it now? If so, there are many other nations who do the same.
But try getting folk to sell anything second hand and the figures quoted here are astronomical and unrealistic. This includes for property even in the current climate. "Ganga" is not a word you see that often! Or try getting people to donate it ...it has to be for a recognised charity because the idea of you earning even a duro, however hard you need to work to sell it, seems to stick in the craw. Unless you are a pukka scrapman, in which case it seems to be fine. (Maybe it is better if it is going to landfill or to be destroyed than to be recycled?)
BUT, as someone else said, get yourself 'adopted' into an extended Spanish family and you get inundated with second hand goods, including clothes that are utterly inappropriate!! You can't sell them as on the next visit the precious broken microwave (we knew you could fix it) will be inspected, admired in its new surroundings, and further items will follow because you are obviously a 'good home'.
It is also a pride and honour thing to be able to support family and not need to seek 'charity'. I think the Brits may have lost all sense of dignity a while back and we are now actually proud of the bargains found in second hand shops. Maybe the inundation of TV progs from US and UK on Spanish TV about the money to be made from "basura", auctions, picking etc might eventually sink in?
The problem I see with actually doing it is that in order to get someone here to trust you sufficiently to sell you, cheaply, the goods in their nave or almacen, you probably need to spend a day or two there, talking, eating, drinking any profit you could have made!