I have had a couple of "experiences" with garages in the last 12 months. I am listing them as replies to this posting so that others can be aware. Hopefully others will share their experiences in the same way.
I one case I don't know if I was conned. Perhaps it was bad luck but if "bad luck" is a regular aspect of that workshop then it isn't just "bad luck" !

Talleres Galindo in Mora
Last year I needed a new shock absorber for the ITV. I chose to have both replaced. I paid for 2 and saw 2 shining threaded bolts on the wheel arches.
A couple of months later I still had a problem and another garage said I needed a new shock absorber. I argued until I was shown underneath the car where there were clearly 2 different shocks with one a lot older than the other.
I had it replaced as I needed to do a long journey the next day. I challenged Galindo's  who claimed they had fitted 2 new shocks and just said (reasonably) that I should have gone to them as soon as I was suspicious.

I use Miguel in Roquettes.Good guy,speaks English and gives you the old parts back.Gives you plenty of options for parts depending on budget.

Once got charged €140 for a service on a lawnmower from the place opposite the park.Took it in for a new cowling underneath near the blades and picked it up 3 weeks (he said 3 days) later.No cowling but bless him,he's done a full service 2.5 hrs work......takes me 20 minutes.Prior to that I'd spent a lot of money with this guy.Not anymore.