For the past 4 years I've more or less lived really close to the neighborhood of Vallila. It's been seen as one of the "up and coming"-neighborhoods of Helsinki and I'm not surprised! It has a lot of young people and an awesome, slightly rugged vibe. It also has quite a variety of flea markets.These are the ones I've visited the most as I used to live in this neighbourhood. I think this is one of the the best parts of Helsinki for some serious second hand shopping.
Mäkelänkatu 34Small place with a good variety of books and especially good quality china. So if you're planning to buy a full set of some china by Arabia, check this place out. It's not overpriced but not the cheapest option. The money they make will go to take care of homeless animals. They also have another shop with more lower quality items and clothes at the headquarters at Itä-Pakila (Yhdyskunnantie 11). For more info, visit: http://www.hesy.fi/elainsuojelukeskus/perintopuoti/ where you an also find other ways to help homeless animals.
A volunteer operated shop with no-profit agenda. All the money goes to support volunteering and for different projects in development aid. It has a christian agenda behind it. Well kept place with relatively good quality stuff and low prices. Wide range of items from clothes to light fixtures but no bigger home appliences. One of my "regulars" and the service has always been good. My best find has been gorgeous cream Max Mara two piece for 15 euros. More info: http://www.emmaushelsinki.fi/fi/kirppu.html
Mäkelänkatu 54 or 56
A flea market with self-service. Pretty messy place to be honest. A lot of stuff though and here you get the real excitement of finding something really nice with a low price. You just have to look past all the clutter first. A real "dig-and-find" flea market. And the owners keep their dog here from time to time so if you're totally scared of them, don't go. Only cash accepted.
Cheap homeware. Not really much to say about this place, quite a "normal" middle-sized second hand shop with some kind of christian charity behind it. Again, quite a range of items, pretty cheap as well. Not very pleasant service from the staff though. I think there are several ones in Helsinki, at least one other on Runerginkatu in Töölö. More information: http://www.samaria.fi/samaria/samaria-shop_kirppikset/
This is a newcomer and oh boy what a newcomer it is! It's a combination between a cafe and a fleamarket, with superb quality on the stuff! Basically it's a normal self-service flea market where people leave their clothes to be sold but the staff here organises it in a really nice and slightly hipsterish way. It seems to be popular among young moms as well. It has had some awesome stuff every time I've been there and it's really easy to get to. You can check out their website on: http://www.vallilanstoori.fi/