My character has some arrows with drow poison on them. I would like to know how much could I sell them for to someone?
Basic Poison-The only poison entry that says anything about number of arrows a dose of poison can cover is the basic poison. 3 arrows per dose.
Cost of arrows-20 arrows for 1gp.
So 3 arrows dosed in Drow Poison is worth 200 and 3/20th of a gp. Assuming you sell them at 50% value, you could get 33 gold and about 2 copper a piece for them.
A table of poisons on page 257 of the DMG gives the market price for one dose of drow poison (the kind that can knock you unconscious for an hour) at 200 gp. If the value of a "dose" corresponds to the unit of poison given for basic poison in the PHB equipment section, a single arrow represents a third of a dose.
That's the price set for player characters to buy drow poison. The book doesn't really treat the issue of selling poison or poisoned objects as treasure, except to make it sound like a rough trade to deal in:
Given their insidious and deadly nature, poisons are illegal in most societies but area favorite tool among assassins, drow, and other evil creatures.
In some settings, strict laws prohibit the possession and use of poison, but a black-market dealer or unscrupulous apothecary might keep a hidden stash. Characters with criminal contacts might be able to acquire poison relatively easily. Other charcters might have to make extensive inquiries and pay bribes before they track down the poison they seek.
With language like this, Dungeon Masters are under a lot of pressure to make it inconvenient or dangerous to deal with poison (I guess because it's so much nastier than simply stabbing people repeatedly).
If your DM lets it happen, though, poisoned arrows would probably fall under the category of "arms, armor, and other equipment" which, per PHB page 144, can be sold for half their market cost.