Li Shenron
Legend
Unfortunately for you, I don't think D&D rules really are on your side.
And it's not that your DM is wrong, he's probably just running the game as it is supposed to be. For instance, it's quite common that you cannot normally sell items at their market price but only at half their price (tho maybe there is not even an actual written rule acout it...).
Anyway to cut it short, if I was your DM I would simply ask you:
Do you really want to roleplay a business for your character, or are you looking forward for a way to have more money than the DMG guidelines?
Because if you answer the first, the solution could be to let you be quite free and compensate perhaps with a little less treasure during the campaign.

And it's not that your DM is wrong, he's probably just running the game as it is supposed to be. For instance, it's quite common that you cannot normally sell items at their market price but only at half their price (tho maybe there is not even an actual written rule acout it...).
Anyway to cut it short, if I was your DM I would simply ask you:
Do you really want to roleplay a business for your character, or are you looking forward for a way to have more money than the DMG guidelines?
Because if you answer the first, the solution could be to let you be quite free and compensate perhaps with a little less treasure during the campaign.