Callais is one of more than 200 "bouquinistes" who sell secondhand books and engravings along a three-kilometre (1.8-mile) stretch of river embankment - a tradition going back centuries that is now under threat from the coronavirus.
While the job was never lucrative Callais, who is campaigning to add the bouquinistes to UNESCO's world heritage list, said the dearth of out-of-towners along the main tourist drag from Notre-Dame cathedral to the Louvre museum had left him struggling to sell anything.
"I sold a book for 16 euros today, I’ve got another customer who’s going to owe me the money later, and that’s a great day for me," he said.
Before the pandemic struck, Callais said a third of his customers were foreign tourists and another third came from elsewhere in France: "We’re totally dependent on tourism."
Even after France's lockdown was lifted in May, many fellow bouquinistes concluded that re-opening was pointless, he added.
In August, the government said the pandemic had cost France up to 40 billion euros ($47 billion) in lost tourism revenues.
It provided some aid for the tourist sector but, as booksellers, the bouquinistes were not eligible. They got some support from a solidarity fund, but that ended in July, a finance ministry official said.
Books have been sold on the banks of the Seine since the 16th century.
Spots are now allocated for five-year periods by the city council. The booksellers pay no rent but must open at least four days a week and, in normal times, bountiful summers would make up for slower sales in the winter.
"Once you’ve sampled life on the riverbank it's a bit like a drug... We come for the human interaction, the exchanges, the sharing of knowledge," Callais said.
"We (also) suffered a lot with competition from internet... companies which sell books but which aren’t booksellers. They’re machines to make cash. There’s no poetry in them."