It really depends on what kind of a game you are running.
I tend to run mine fast and loose and narrative-driven - I have no problem with my ogres being quantum, so long as the player won't notice it. If they go down the wrong corridor first, I'm happy to flip my map so that the encounter that needs to be first, is first. But if they make informed decisions (if, to paraphrase @SirTechSpec's excellent answer, they avoid the corridor littered with bones and go down the one littered with flowers) then I can't teleport the ogre, and I'm gonna have to make sure the flower-fairies work as a first encounter, instead.
That the players face a good level of challenge - dying rarely, permadeath almost never, but having the sensation of threat of TPK at least once every few sessions - is more important to me than whether they got the right price on something.
I'll explain this approach with an example.
So, sure, I'd have them get a cart by selling a barge, but ideally I'd handle that at a high level, just having the harbormaster offer a trade-in.
But since players never do what I expect, they sell it to the wrong guy and get ripped off. As others said, I'd have them find out they were shortchanged... but then have them hear that the trader is coming after them for his 5pp back and then some, with the town guard backing him up, because the thing sank in the harbor as soon as he loaded the cargo, and don't you know that selling a lemon's considered a very serious crime in these parts?
That news should give them the impetus they need to get the heck out of town and find a way to travel fast, at least until they've left the reach of the town guard.
But since players never do what I expect, they might passively let themselves end up in the cells. So the trader might be willing to cut them a deal. There's a package they could deliver for him that might just cover his losses... "Just take this package to the inn three towns over, and the recipient will make himself known. Yes, it's a coffin. Yes, there's a body in it. You're in a jail cell, I'm offering you a way out, and a cart to carry it on, will you really complain about the smell at this point? And if you're gonna do this, you need to get going. The day's hot, but Eddy the Embalmer's not, if you get the drift."
But since players never do what I expect, they push back, say they needed to travel at night to preserve the body, then send the thief to check if the sunken cargo was deliberately scuttled; so I'd have to decide on the fly whether it was or not. The players won't typically realize how deeply they've descended into a freeform adventure at this point, and will assume that it's all plotted out and railroaded for them, and I'll support that by dropping details for verisimilitude. "The watchman at the docks is Old Jack. A well-worn battleaxe on the wall over his head suggests he might actually have a couple levels, but he and his little terrier, Young Jack, both seem to be napping comfortably - they likely don't get a lot of traffic this time of day."
And since players never do what I expect, my goal once they go off the rails - which I encourage - is to give clues and cues to lure them to their destination, or at least a waypoint towards it. So I'd either have the cargo legitimately sunk, or have signs that it was deliberately sabotaged by Old Jack, at the behest of some guy two towns over in the direction they were going anyway, because of a deal that'll be going down soon...
And so the plot moves on. When in doubt, kill an NPC. "A crossbow bolt whistles out of the darkness and nails Young Jack to the boards. There's a note attached. Old Jack starts awake and looks around for what woke him - you have a couple seconds to act before he spots you: what do you do?"
TL;DR: Crank up the drama and intrigue and fighting, crank down the bookkeeping, and roll with the punches when they drift off track. I presume-but-verify that players honestly keep track of every copper, but I should rarely need to know how much cash they're carrying.