After the council voted to approve the decision, campaigners had a three-month deadline to file a judicial review - due as ATG went to press. Thanks to an anonymous £10,000 donation, a community group planned to do exactly that.
A website has been set up to help cover legal costs which could rise to £20,000.
Croydon chose Christie's as the auctioneers for its sale of 24 Chinese works of art from the collection. The works, which feature Ming, Qing and earlier ceramics, were acquired by local businessman and collector Raymond Riesco (1877-1964) before entering the council's collection on his death.
The council aims to use the funds raised to help redevelop the Fairfield Halls entertainment venue and Christie's will now offer the items for sale in Hong Kong on November 27, and expect to make over HK$113m (£9m).
The protestors have launched a petition and have a Facebook site.
Steven Downes, editor of the InsideCroydonblog, told Museums Journal: "We have to raise a serious five-figure sum so the concerns of the local community, who don't want this collection sold in a pawn-shop sale pushed through by the council, can be properly and legally dealt with."
The judicial review would be based on a challenge that the sale goes against council policy.
The rest of the Riesco collection, comprising 206 other works spanning the Neolithic period to the 19th century, will remain in the Riesco Gallery in the Croydon Clocktower where they are on free public view.
Croydon council recently announced it had resigned from the Museums Association after the organisation's criticism of its Riesco decision.