For reasons too complex and stupid to go into, I've recently been looking at item values and the 5e currency system. As I understand it, 1pp = 10gp = 20ep = 100sp = 1000cp, which places 1cp as the smallest representable value. However, the price per unit of Sling Bullets is 1/0.2cp (source: PHB page 150). With that in mind, how would a particularly difficult player go about purchasing a single Sling Bullet? Break a copper piece into fifths, perhaps?
Thanks for the answers, they're very helpful – for DMs. I didn't put this in the question originally because I didn't think it was important, but I can see now that was wrong. What I'm actually doing is building a relational database for D&D objects. So, when inserting a row into the item table for Sling Bullets, I wanted to follow the example of Arrows and Crossbow Bolts and put them in as a single object with a single value, although in the PHB itself only a price for Sling Bullets (20) is given.
Given the answers, I'm going to move ahead by only listing the value for an amount of an item that is at least 1cp (Sling Bullets (5) in this case). I'm going to accept an answer based on the fact that it lead me to this resolution by explaining what would be most helpful to a DM. However, if this new information leads to new suggestions, I'm open to those as well.
Best Answer
As a DM, you aren't required to obey the strict prices set in the PHB. The prices listed are mostly just used as guides for creating characters on a budget; once you're in the adventuring world, goods cost whatever the local shops value them at, with the PHB listed prices as suggestions for "fair market" prices.
That being said, there's any number of ways to approach this. You could introduce a new currency that's worth 1/5 or 1/10 the value of a copper piece to give a single divisible currency for handling especially cheap goods.
Personally, I'd probably just rule that any shop that sells Sling Bullets doesn't sell them in packs of less than 5, for 1 copper piece for 5. That way, you don't have to introduce a new currency.
If the player is genuinely only interested in buying the single unit, I'd probably just sell 1 bullet for 1 copper piece. The scenarios where this would seriously inconvenience a player are pretty esoteric.