"Look, all I do is take a little from people who have too much and then I spread it around a bit. I help to keep the economy in balance."
"But if that piece of jethryk is so valuable, why don't you just sell it? Then you'd have plenty to give to those who need it."
"Oh, I don't think its worth that much."
But why doesn't he think it's "worth that much"? His whole plan is predicated upon the idea that the Graff will want to buy Ribos because it possesses a rich seam of, in the Graff's words, "the rarest and most valuable mineral in the galaxy"!
There's only one explanation. Differing conceptions of value.
The Graff wants the jethryk because he can sell it and buy ships, soldiers, weapons, etc (ie for its exchange value). The Doctor and Romana want the jethryk because it's also the first segment of the Key to Time (ie it has an intrinsic use value). Garron wants it because its a means to an end. He could sell it and live off the proceeds... but then he'd die of boredom, and someone like the Graff would have the use of it. And Garron wants to keep using it. Like the Doctor, he thinks of it as a use value, not a commodity.
It's part of his con game, you see. It's how he snares men like the Graff. It's how he runs rings around them, fools them, bilks them, makes them look like the prannies he thinks they are. This is a poor boy from Hackney Wick. This is how he gets his revenge on the big shots. This is how he brings them down to his level.
For all his flaws, he's a leveller.