It's "Cannabis" on this side of the pond, thank you!
One set of scales is enough as long as the resolution is sufficient: you just need different weights to balance them with (though something suitable for weighing several ounces is not likely to be accurate for fractions of a gram). Copper coins make a handy measure for parts of an ounce: 1p is 1/8oz, 2p = 1/4 (and save you getting nicked by Trading Standards since it's hard for them to prove you were dealing in illegal measures :-)).
And 'poor' is not necessarily the most appropriate adjective for dealers (though, like plumbers, it's not necessarily the licence to print money that the Bile Duct & Co would have us believe).
Disclaimer: "or so I've heard" :-)