I have all the items, but Delvin doesn't have a interaction option to buy them. I have done all the Thieves Guild quests except becoming the new Guild Master. Do I have to become Guild Master for him to buy them?

I have that problem too. Apparently after you kill Mercer he does not want to buy your stuff.

Which makes no sense because you get one of the items off Mercer's dead body. Have you done everything to become the Guild Master?