I wouldn't quite implement game play as an accurate representation of how things would work if put into a more narrative format. If that were the case, no one would be suffering much aversive effects from skooma, at least not for more than a short period of a few seconds or so (If going by Oblivion) or outright beneficial with no strings attached (Like how Skooma is just a cheap energy potion in Skyrim). Beggers in leyawin might not actually be buying whole bottles of skooma; it's likely either being watered down but paid for in full price, or even just given in smaller quantities. Bottles of skooma in Oblivion are [i]tiny[/i] compared to what's being used in Skyrim. 

Besides, we're not selling it to beggar's anyways. We will be, if only for that particular job, suppliers for the dealer, not the dealers themselves. Now I can't say for sure what Commander has in mind, but I'm going to take a good guess that Sharee didn't just have us take the Skooma because it seemed like the most valuable thing at the time. I'm sure she has a buyer who'd purchase it for a fair price, instead of say, just any old dealer at a dock in Blackmarsh. Who himself would likely profit larger from the sells with some trickery like spiking the Skooma or some such, but how he sells doesn't matter; only that Sharee gets the skooma to someone who won't give her a bad deal.