Certain shops have different things available. For example, Sha Enin in NCR deals in Melee weapons as one of his types of buy-able items.
This means that you will always be able to sell a Sharpened Spear to Sha Enin for 90 caps.
On the other hand, Dr. Jubilee in NCR only deals with Medical Supplies. That means that if you have high Barter, you will be able to get a higher price, and if low barter, a lower price than 90 caps.
It's mostly to prevent Barter alts.