If you are prepared to travel onto the delta for your wood there is an excellent lumber yard on the main raod (as opposed to the bypass) between Deltebre La Cava and Deltebre Jesus & Maria.  If you are heading from the former to the latter it is on your right in a quite built up area.  (Its hard to know where one Deltebre starts and the other finishes).  As far as I know there are no English speakers there but having explained what you want form the guys on the ground they give you a document saying what you've had and you go upstairs to the office and pay.  There is a certain element of faith in this as the guys on the ground don't get involved in pricing - and there are no prices displayed anywhere that I've seen.  Having said that I've not been upset by what I was charged.  The office staff refer to a price list rather than 'inventing' a price based on your nationality.
Hope this is of use.
Mike
PS  I have heard other people getting diddled at the builders' mercahnt that you refer to (but the plumbers' merchant next door is very good).