seeth no man Gonzaga his heirs and his concubines (11)
To really get the gist of this line, you need to know that Pound is referring to Francesco Gonzaga, a rich dude who lived in Mantua (that's in Italy) back in the old days. Here, Pound is implying that in ye olden times, people like Gonzaga were willing to pay artists to create great art without any expectation that they would make money off the art some day. The idea of having something beautiful was way more important than having money. Pound wishes people still felt that way.
no picture is made to endure nor to live with but it is made to sell and sell quickly (12-13)
Unfortunately, artists nowadays don't always have some rich patron to take care of them. Instead, they have to create art that they can make money off of if they plan on oh, you know, feeding themselves. For Pound, though, this is a terrible part of modern life, because it encourages greed in artists, who can't do their work well if they're thinking about selling it to some ordinary Joe. Beauty needs to be their first priority. In other words, Pound wishes there were still rich patrons who would pay artists to create beautiful work.
Pietro Lombardo came not by usura (28-29)
Pound offers us a lengthy list of people who never would have become great artists in a world with moneylending and modern finance. In other words, he says that great artists would never have existed if the old world had chased money the same way the modern one does. Beautiful art is completely incompatible with a desire to make profits, and the modern world always tends to side with profit over beauty.
Not by usura St Trophime Not by usura Saint Hilaire (35-36)
After he's listed the great artists who couldn't exist in a world with usura, Pound then claims that beautiful buildings like St Trophime and Saint Hilaire never would have been built in the modern age, either. If you take a look at modern buildings, they're usually built out of cheap materials instead of pure stone, and they aren't covered with a ton of fancy statues. But stuff that's beautiful doesn't always care about how much things cost. Just take a look at this modern church, which Pound would no doubt be proud to see. Whoever got this thing built definitely wasn't concerned about how much it cost.