To be fair the decent ones are around £80-90 but i'd expect it to come with a pressure dial for that sort of money. They are designed for the glazing industry and hold about 95kg each. Basically you need the pressure dial when carrying that sort of weight as if it's that heavy it's normally a bloody expensive piece of glass so you want an early warning system to tell you if it's about to fail.
So yes they can hold a heavy tile absolutely no problem but only if it's perfectly smooth. I'm pretty sure everyone knows how they work which is why i'm guessing you asked him to pick up stone with it as of course it was going to fail!! Salesmen reading the specs without understanding what they are trying to sell