Do you see that "0" above your catch number? That indicates how much you can carry if that number = 0. You cannot buy anything from NPCs.
You have too much weight on you, it must be a few items you have that has weight as I said before, check your whole bag and look through your items to see if they have weight. Remove them, put them your locker and your problem should be solved. If it's not then I have no clue what can be causing this.
Example:
15:38 You see a voodoo skull.
It weighs 10.00 oz.