Bukkit

ERSCommands comes with sign manipulation. You can add colors with & and the prefix (a-f, o, k, m, n, l, r, 0-9). Or make buy and sell signs by putting:

[Buy]

<Quantity>

<block/item>

<price>

It's the same with sell signs:

[Sell]

<Quantity>

<block/item>

<price>