Earlier today, I bought some postcards in Barcelona, from a gift shop at a museum which didn't sell stamps. I then retired to a nearby cafe for a coffee, and wrote the postcards. After that, I hopped on the metro to Barcelona Sants station, and utterly failed to find anywhere to sell me any stamps for them, and I couldn't even find a postbox...
For me this in now moot, as my TGV has whisked me across the border, and consequently these postcards will have to have French stamps instead. However, for next time, I'm left with the question - is it possible in Barcelona Sants station to buy stamps then post letters? Or failing that, how nearby would one have to venture to do so?