Why doesn't this sell the gun?

The reason why is because you made it detect it lowercase so this is how it would be if you wanted it lowercase:

function onChatted(msg, recipient, speaker)   

    local source = speaker.Name

    msg = string.lower(msg) 

    if string.sub(msg,1,5) == "sell " then 

        if workspace:findFirstChild(speaker.Name) == nil then return end
        if workspace:findFirstChild(speaker.Name).Humanoid.Health <= 0 then return end

        if string.sub(msg,6) == "gun" then --Change this for every gun name.
            if speaker.Backpack:findFirstChild("Gun") ~= nil then
                speaker.Backpack.Gun:destroy()
                speaker.leaderstats.cash.Value = speaker.leaderstats.cash.Value+50 
            end
        end
        end
        end
function onPlayerEntered(newPlayer) 
    newPlayer.Chatted:connect(function(msg, recipient) onChatted(msg, recipient, newPlayer) end) 
end  


game.Players.PlayerAdded:connect(onPlayerEntered)