Sorry BB, took longer to get back to this than I meant to...

I think everything you've said in your last comment makes sense. Definitely related to stock trading and such.

Here's a stab at the missing stanza:

I had a friend who met his end, 'twas easy as a pie
He thought he'd let the shares he bought ascend -- oh, to nigh
But the banksy I had brokered couldn't quite be eye to eye
And now he's left a beggar in Throckmorton

Some comments -- the n in "Oh, to nigh" has more of an L sound than an N but I couldn't think of anything else logical. It could be ascension instead of ascend, or perhaps something else.

I've found no evidence "banksy" has ever been used as a slang term for a banker, but it makes sense... some, at least.