The problem with waste feels more concrete in Spain than anywhere else I've lived, probably because you actually see trash everywhere as people simply dump it on the streets and wait for the trashman to pick it up. Recycling is not the country's strongest point either - I'm still especially bothered by the lack of proper recycling for used plastic bottles and cans (e.g. in Finland people actually get paid by returning the bottles and cans back to the supermarket).
However, what I do like here more and more is the positive attitude towards second hand shopping and ecological lifestyle. With hundreds of second hand shops, markets and events, Barcelona is a mecca for vintage hunters, flea market enthusiasts and all sort of eco-friendly hipsters. Whatever it is that you are looking for, I'd say there's always an option to buy it second hand. Apart from nature, it saves you money and everyone thinks you're cool!
Where to start then?
Loquo.es and segundamano.es
I've bought furniture and heaters, sold purses and electronics, and swapped some other stuff through these portals. You can find literally anything with a really good price, if you are fast - the best stuff is sold in minutes. I've had especially good experience with Loquo's home section; there's a lot of short-term expats selling their semi-new furniture in half price. To guarantee that you actually get what you want, I recommend calling the seller immediately instead of emailing and going to pick up the items as soon as possible. I never ended up to any scam, so I would say both of the portals work well.
I know, the thought of it may sound weird, but actually it's crazy what you can find on the streets in Barcelona! All the neighborhoods (barrios) have agreed weekdays when inhabitants are allowed to dump on the streets - in addition to the usual trash - whatever they like between 8pm and 10pm. Meaning you can find sofas, chairs, tables, TVs, beds, you name it, and all for free. I think this is a brilliant idea and really reduces the ecological footprint! I found an article where you can see the schedule of different neighborhoods (it's in Catalan, but see the map on the right and use Google translator :-) ).
Barcelona's biggest flea market Els Encants takes place on Monday, Wednesday, Friday and Saturday from 8am till 3pm close to Glories metro station and offers everything from beds to bras and radios. It's not only old items they sell there - in my opinion, there's a lot of crap for sale but by being patient and taking a closer look, you can find some real bargains. Beware of the pickpockets though!
There are several smaller flea markets around every once in a while, those what I would call "real" flea markets. For example, today I visited one in Raval where people were selling mainly clothes and accessories. There's also one organized twice a year in Barceloneta, which I absolutely adore - just can't remember the name now! Finding these events can sometimes be tricky, but Facebook is a good channel to keep yourself up to date. Check out this page , it has turned out to be quite useful.
There are various vintage and fashion events going on almost every week. Depending on the nature of the event, they usually offer a mixture of second hand items (mainly clothes and accessories) from professional vintage storekeepers or individuals with interest in this area, and pieces of young designers' collections. One of the nicest markets I've been to was After Tea, which is, according to my understanding, organized more or less regularly. Same advice with these ones, just look them up in Facebook and subscribe to updates in order to know when&where.