I didn’t jump to answer this question because, although I personally know many of the estate agents in Barcelona who tend to deal with foreign clients, I’ve never actually purchased a property through any of them. That said, there’s only one I know of I wouldn’t go near.
Buying a home in Barcelona is low risk if you do a proper legal search and use a good lawyer. Because the market is deep and liquid, it’s quite easy to get a feel for market prices (using one of the big portals like Idealista or Fotocasa or Habitaclia). So where are the risks? Buying something illegal? That’s very unusual in Barcelona – this is not rural Andalusia. Overpaying? It’s hard to overpay in this day and age if you do a bit of homework. Mind you, there’s a portal called Kyero that claims the average price of property in Barcelona in Q1 2015 was €588,000, so you do have to be careful about how you interpret the data available ;-).
For what it’s worth, I do have a page for people looking for estate agents in Barcelona. I have no financial interest in this, I don’t get paid by any estate agents for anything. Please read the disclaimer!