Browsing classifieds site Le Bon Coin, Ziani thought he was buying an unsigned piece by 18th-century artist Vernet, and offered €700 for the work.
But once the painting arrived, Ziani's 11-year-old son spotted the almost illegible signature 'A. Renoir' and date of 1864 in the corner of the 96cm by 76cm painting, Le Progres reports.
After reading a Renoir biography, Ziani has identified the piece as Soir d'Eté (or "Summer Night" in English), painted by a 23-year-old Renoir before he had developed his distinctive impressionist style.
Soir d'Eté was exhibited at the Salon de Paris in 1865 along with one other Renoir work, Portrait of William Sisley, which is now owned by the Musee d'Orsay. Since this public display, Soir d'Eté has been listed as a 'missing work' by France's National Art Institute.
A classification number is visible on the back of Ziani's painting, but the National Museums' Archive's records from after 1853 are missing, so it is not possible to see what this number corresponds to.
Ziani has now sent the artwork to a specialist lab in Bordeaux to work out whether it is genuine.
Researchers so far have confirmed that the pigments in the painting are compatible with other Renoir pieces from the period, and the thin canvas and framework are similar to those preferred by the painter, but it could take months for them to come to a judgment as to its authenticity, particularly as there is very little information about Soir d'Eté.
It's been a good year for long-lost art in France. Back in April, a family clearing out the attic of their house in Toulouse to try to fix a leaky roof stumbled across what is thought by many to be a genuine Caravaggio painting. It was classified as a 'National Treasure', which means there is a 30-month ban on it being exported, until experts can determine its authenticity.
If this painting turns out to be authentic, Ziani could be in for a serious payout. Another painting by Renoir, Bal au moulin de la Galette, became one of the most expensive paintings in the world when it was sold for $78 million at Sotheby's in 1990.