Unless this is some bug I'm unaware of...all merchants/traders have a limit to the amount of caps they have on hand at any given time. If you max out the value that they can pay you for what you're selling, you have to either do some more trading (such as buy something from them) or wait however many days it takes for the merchant to refresh his/her inventory and caps. The Vault wiki should tell you how often this happens for each merchant. Moira restocks on Sundays and Wednesdays, or something.