跑商：两城差价的低买高卖

; trade_goods.txt 跑商货价
丝绸   北城=100 南城=160
瓷器   北城=200 南城=260
铁料   北城=150 南城=140

local function buyGoods(actor, goods, unit, n)
    setplayvar(actor, "cargo_" .. goods,
        (getplayvar(actor, "cargo_" .. goods) or 0) + n)
    sendmsg(actor, 1, "购入 " .. goods .. " x" .. n
        .. "，单价 " .. unit)
end
local function sellGoods(actor, goods, unit, n)
    local have = getplayvar(actor, "cargo_" .. goods) or 0
    if have < n then
        sendmsg(actor, 1, "货不够，现存 " .. have)
        return
    end
    setplayvar(actor, "cargo_" .. goods, have - n)
    giveitem(actor, 9021, unit * n)
    sendmsg(actor, 1, "售出 " .. goods .. " x" .. n)
end