An original translation
Here I said, “What? Do you know the sort of risk you’re running by gambling your soul? Look, if you had to hand over your body to someone and run the risk for better or worse, you’d look long and hard into whether you should do it or not. You’d call your friends and family together for advice for days on end to figure it out. But as for what you rate higher than your body – your soul – on which your success or failure entirely depends, as it turns to better or worse – about this, do you bother to consult your father or your brother or a single one of your friends about whether you should hand over your soul or not to this stranger who just turned up? No, as you say, you only found out last night and you’ve come this morning, without hearing argument or advice about it, ready to spend your own money and your friends’ money, since you’ve already figured out that you absolutely have to spend time with Protagoras, whom you don’t know and haven’t ever spoken to, as you admit. And you call the person you’re about to hand over your soul to a sophist, but you clearly don’t know what that is.”
When he heard this, he replied, “Well, it seems to be as you say, Socrates.”
“So, Hippocrates, maybe the sophist is a kind of shopkeeper or a retailer of stuff which keeps the soul fed? Because that’s the kind of person he seems to me.”
“But what does the soul feed on, Socrates?”
“On lessons, I suppose. But watch out, so the sophist won’t deceive us when he praises what he sells, just like a shopkeeper or a peddler who sells food for the body might. In fact, these people don’t even know themselves which of their products is better or worse for the body, but they praise everything they sell. And their customers don’t know either, unless they happen to be a fitness or medical expert. The same goes for those who go city to city, selling and hawking their lessons to anyone who’s interested. While they praise everything they sell, my friend, some of them probably don’t know whether their stuff is good or bad for the soul. Their customers are also ignorant unless, again, they happen to be doctors for the soul. So, if you somehow know which of their lessons is good or bad for you, it’s safe for you to buy from Protagoras or anyone else.
Plato's Protagoras, a translation 
“So, Hippocrates, maybe the sophist is a kind of shopkeeper or a retailer of stuff on which the soul is fed? Because that’s the kind of person he seems to me.”
“But what does the soul feed on, Socrates?”
“On lessons, I suppose.” — I like this part and the tone. Reminds me, for some reason, of Lewis Carroll. The middle of the speech on gambling, though, feels to me somewhat muddled (“But as for what you rate higher than your body – your soul – on which your success or failure entirely depends, as it turns to better or worse – about this, do you bother to consult your father or your brother or a single one of your friends about whether you should hand over your soul or not to this stranger who just turned up?”). These Greeks could talk fast, I guess. Though I follow Socrates here, he sounds wordy.
Striking, isn’t it! That sentence takes up almost half a Stephanus page; it’s the longest one I’ve come across in Plato, but there’s no anacolouthon, and it makes perfect sense.
The ‘would’ in the last sentence surely requires a ‘knew’ in the antecedent of the conditional?
And in ‘Their customers are also ignorant unless they again happen to be doctors for the soul’ — I think ‘again’ needs to be elsewhere in the sentence. Perhaps ‘unless, again, they happen to be’, or ‘… ignorant, again, unless they happen to be…’
I think I meant to change the sentence from “If you knew, it would” to “If you know, it is” but only did so partway. Will fix.
Spot on. Again, hapless editing.
A question about πωλοῦσιν (and cognates). First, is ‘goods’ common in standard American usage outside of economics or in idioms like ‘deliver the goods’ etc. I ask because the bits that go ‘are these goods good or bad’ creates wordplay where there’s none in the Greek, or perhaps draws attention to the economic sense of a good as opposed to an (economic) bad, which makes the rest of the sentence seem like an oxymoron.
I quite like ‘wares’ myself, but that’s not much more American. But I like ‘stuff’ for ἀγωγίμων earlier on. You could use it again, or maybe ‘products’, or ‘merchandise’.