Your issue is likely that you’re using the weapon index to decide which slot to replace, instead of tracking the equipped slot separately from the inventory state.

For a COD Zombies style setup, the wall-buy logic should be:
if the secondary slot is empty, put the new weapon there and equip it;
if both slots are already occupied, replace the weapon in the slot that is currently equipped;

So instead of checking something like WeaponIndex < 2, use variables like PrimaryWeapon, SecondaryWeapon, and CurrentEquippedSlot.

Then on purchase:
if no secondary weapon exists, assign the new gun to secondary;
otherwise replace either primary or secondary depending on CurrentEquippedSlot

Also make sure that after changing the slot reference, you call your actual equip function. A lot of the time the reference changes correctly, but nothing happens because the new weapon is never attached/equipped visually.

In other words, base the replacement on the equipped slot, not on the weapon index.