M0nica
I would have spoken very loudly and told the young man to put the goods back. Other people would have turned to look at him, and you could, if you had your phone with youhave taken a photo of him or pretended to.
With a bit of luck, he would have dropped his loot and run
I'm with M0nica on this one - indeed, I have done exactly that more than once, saying VERY loudly, "Excuse me; are you planning to leave without paying for that/these?" On two of the occasions, the individuals (one male; one female) put the item(s) back; the other time, the male shoplifter ran out of the shop with the goods. I have also stepped in to stop someone stealing (of all things) an advertisement placard from a London Underground train - whilst everyone else sat mutely and watched. In the middle of Peterborough a few years ago, I ran into the road to drag someone off beating an old man to a pulp (literally: his face had the texture of a jelly), and one evening on a London bus (No 38) in Islington, myself and another passenger intervened to stop a drunken bully dragging his wife/partner on her back down the aisle by her hair. On both these occasions, absolutely no-one else raised a finger to assist - other than dialling 999 at my shouted request. Both times, the police turned up in time to arrest the offenders - who, for whatever reason, stayed around the scene.
Everyone has to make their own judgement about what they are going to do when faced with these types of things, but I believe strongly that "do nothing; it's not my concern" should not be the default answer.