Think through the logistics of this. It's heavy as fuck, so you're going to need some kind of vehicle so you can carry enough to make the reward remotely worth the risk. So now it's not just your face you have to hide but the registration of your vehicle, as the car park likely has ANPR. It will also take a while to load up so you probably want a team of people so you can do it quick enough to get away before whoever s monitoring the cctv spots what's going on and calls the police to tell them there's a gang of masked men loading up a van (which they can give a detailed description of including the reg or point out the reg is covered, so even if it's an unmarked white van you're easy to spot) and you've now got several people who might blab. Then you have to find somebody willing to buy it in bulk or somewhere to hide it whilst you sell it off at a couple of quid a time. Police will be aware of the crime and it won't take long for them to catch you if you're going around pubs shotting bags of bark and compost.

It might vaguely make sense if you had a specific need and were willing to travel to the other side of the country to do it (because you can guarantee police will be asking questions of anyone in the area doing a garden renovation or local landscapers so suddenly appearing with a load of this stuff you can't explain where it came from will be a problem) and were well organised enough to fill up the van, get away and switch plates quickly and slickly enough you didn't get spotted on the way back but at that point, is it really worth the bother for a low value haul?

You'd genuinely be better off selling class As, much better margin and almost certainly lower risk than this.