The poor Man's Farthing is worth more Than all the Gold on Afric's Shore.
The poor man's farthing (just a quarter of a penny in the old British currency system) is worth more than all the gold on the African shore ("Afric" is the same as Africa—in case that wasn't clear) because it's worth more to him. Just like people say "Beauty is in the eye of the beholder," Blake's saying that value is in the eye of the beholder, too.
This shows how much importance Blake gave to the imagination. For him, the imagination is the power that shapes reality. If you take it away, you don't really have anything—except for maybe chaos. But the imagination can make even an almost worthless piece of money worth something real (if you need it badly enough).
Also, for the poor man, this farthing might be the difference between life and death, might get him someone's leftover crust when he'd otherwise starve. He needs it more than the colonialists who are trying to get that African gold.
One Mite wrung from the Labrer's hands Shall buy & sell the Miser's lands: Or, if protected from on high, Does that whole Nation sell & buy.
This continues with the currency theme from the last couplet. This time, Blake's saying that the tiniest amount of money a worker earns ("One Mite") is, ultimately, enough to buy and sell all the land owned by a rich miser.
Is this true?
Since a miser, by definition, just sits around and doesn't create anything or do anything with his or her money, a worker has more power because his or her money actually stands for something—it symbolizes the work, the creative activity. Also, there are a lot more workers than there are misers—giving them strength in numbers.
The tiny amount of money they earn is worth more, too, because they value it more than the miser does, in a way. The fact that it's earned makes it valuable.
If the government will protect the money its workers earn, says Blake, making sure they're not getting ripped off and that no one unfairly reduces their wages, the power of that money will be enough to control the entire nation.
As a fan of democracy and a secret opponent of the monarchy, Blake thinks that having a decent amount of money will eventually help the workers get more power—they'll have the most say in how the country is run.
(Since democracy did grow more powerful in England, and workers increasingly won more rights, Blake was actually pretty prophetic here.)