Dexter owned a modestly prosperous glove and leather-dressing shop in Newburyport before and during the Revolution. Somehow, he managed to hang onto enough hard currency to be able to buy up Continental currency some time after the war ended. Massachusetts and the Continental Congress had both borrowed heavily, and hard money, specie, was scarce. There had been a revolt ( Shay's Rebellion) when the government in Massachusetts tried to wring hard money for taxes out of its population- many of the farmers of western MA seemed to have been simply trading in notes ( like IOU's) and avoiding regular commerce. Dexter was therefore able to buy apparently worthless Continental currency at an incredibly low price, and when the new Federal government, with the urging of Alexander Hamilton, actually redeemed it in 1790 for a small percentage of its face value, Dexter became wealthy. With that wealth, he began to speculate, likely on the cargoes coming into the port of his town, striding down to the wharves every time a ship was in, and looking them over. But what those speculations really were will never be known: though he left some writings, his papers and business records have not survived. Certainly, some must have been invented and ascribed to him later.And, notice that these stories have an echo of his great score with Continental currency- "they laughed when I said I was going to do this deal, but then..." But Dexter would have been like many businessmen in bragging about his successes- even inventing some himself- and keeping quiet about his failures. Did he send coal to Newcastle and sell at a profit, because of a miner's strike? At the time when such labor actions ( like that of the Tolpuddle Martyrs) resulted in quick transportation to Australia, that seems dubious. So ,too, the idea that warming pans, shipped to Jamaica, could be employed in the molasses industry- they're pretty lightweight, for using as ladles. But his buying up a cargo of whalebone ( balleen) and selling it to corset makers is quite possible. He does not seem to have died immensely wealthy, so likely he had some good luck and some bad. But his odd writing style is not an indication that he was stupid. He somewhat resembles in that the English banker Jemmy Wood, almost a contemporary of Dexter, who looked silly, spoke oddly, but made money nonetheless.

Because Dexter left no papers, only a few writings, it's unlikely that we'll get good sources for him- unless his account books are found somewhere- and so these stories are likely to have to just be judged on their probability. Accounts of Dexter online seem to be a mess: if you go to Wikipedia, you'll find the article on Dexter has so many CITATION NEEDED notes that if they were lights, it'd be a Christmas tree. Author John P Marquand, whose family were an established part of Newburyport, Dexter's town, wrote two books on Dexter. The second , Timothy Dexter Revisited, made a decent attempt to sort things out ,and at least narrowed down the possibilities. It is garrulous on the subject of the region's history and probably not too reliable on that, but in spite of this embroidery it's reasonably careful about Dexter. You can also borrow it on the Internet Archive.