Sue him?
The item must be VERY expensive, as Gumtree deals go,
for it to make business/commercial/economic sense
to sue anyone over a Gumtree deal.
Unless you (the seller) is operating a business (lawyers say "operating in trade or commerce"),
then this is probably not an Australian Consumer Law question,
because it's not a transaction to which that law applies.
It seems more like a simple question of contract.
Basically, a deal, once done, is a deal.
In that sense, your husband had already sold the item to the first guy,
(even if the guy had not yet handed over the money) and,
on that basis, could not validly accept an offer from the "someone else".