Green’s Dictionary of Slang

1. a seller of stolen or inferior clothes, or of material, offered as superior products.

1922		Argus (Melbourne) 24 June 22/1: ‘Shoddy dropping,’ or ‘dud-dropping,’ the Bench at the City Court learned yesterday, is buying cheap suit lengths and cloth and hawking them about the city.
1939		Cloncurry Advocate (Qld) 9 June 1/1/4: The suit lengths of course are fresh from the warehouse down the street. [...] some of the smaller warehouses have rubber stamps and with these they mark the ‘dud-droppers’ stock in trade with whatever English brand he fan cies.
1946		Truth (Sydney) 30 June 30/1: The hoary suit-length racket with which pedlars have fleeced the innocent for 20 years or more is flourishing again. Pedlars, who are known to those who live by their wits as 'dud droppers,' buy Australian suitings for about £2 10s a 3¼ yard length and sell the stuff at skyrocket prices.
1984		Macquarie Dict. Colloq. 104: dud-dropper n. one who sells inferior goods as superior goods.

2. (Aus.) synon. with ring dropping n. (1)

1925

Argus (Melbourne) 29 Apr. 9/5: ‘Dud-dropping.’ Selling brass rings for gold.

3. a confidence trickster specializing in selling otherwise second-rate goods to those who believe that they have ‘fallen off the back of a lorry’.

2002

Larry’s Aussie Sl. and Phrase Dict. 🌐 Dud-dropper one who sells cheap stuff as good stuff, implying that it’s cheap because it fell off the back of a truck.