Friday the 13th. Itchy Art Basel buyers won’t walk under a black ladder or even near a hack auctioneer.

Like houses of prostitution, galleries just wait for a spender to walk in. The deal is, with nouveau art collectors, all that’s cheap is the money.

“Amazing Grace,” a 1952 Thanksgiving painting Norman Rockwell originally earned a bagel for, brought $46 mil. A Francis Bacon grabbed a few guineas over $142 million. Warhol’s “Silver Crash” went for $105 million. My friend declined $10 mil for her Warhol, saying: “Chinese will pay twice that.”

Miami’s trash-to-treasure hustle is because galleries are in one place. Collectors needn’t schlep to 100 locations — Madison Avenue, London, Geneva, LA — to see something.

The rule is they don’t do layaways. No putting $30 down to hold a Jackson Pollock until you cobble together another few million. Those 110 private jets that hit Basel in Switzerland, where the original art show was held, had to arrive Day 1, early, as if it’s a cellphone sale on Black Friday, then jump around like a Music Hall Rockette. Billionaires must rush in, squeeze through, see everything. Or save that time and just grab a cab downtown to stare at a Banksy on the side of a fence.

Also, need money? Not so easy to unload your $6 million house. Yes, you can dump your Rolex, Ferrari or airplane for liquidity. In days of yore, it was jewelry. Old-time Cubans escaped with gems pinned to their drawers. On West 47th Street, they’d unload that emerald not worn since Batista’s inaugural.

But jewelry hasn’t kept pace with the economy. Remember the Cartier building — now worth maybe $50 mil — changing hands over a pearl necklace?

Like high-class pawn brokers, today’s auction houses self-insure. Charge a percentage of the worth. And advance you the money.

But there’s more paperwork than for entrance to Harvard: The sales tax. The commission. The insurance. The buyer’s premium. Legal fees. Appraisals. For written contracts. What if the piece turns out to be fraudulent and you’re really stuck with an Irving Renoir? Who’s the original owner?

One newcomer collector, asked how he got started, replied: “I needed something to hang in my living room. And the prices were too good.”

There are different ways to get into the game. First, a conditioned report so you’re sure a 1912 Picasso is in good condition. A reputed dealer. Biggies take 50 percent. Sell something for $9 mil, a first dealer like a Larry Gagosian will cop $4 ¹/₂ of that. Then you can ask him how’s his $55 mil Boeing Business jet doing.

New guys with hot wallets roam around. Some Kentuckian nobody noticed before bought an Italian installation compiled of a stack of bricks. OK? We’re not talking Rembrandt here. Not even one of those multimillion-dollar dogs by Jeff Koons, who says “Art is like food.” Buying a bust made from a pile of Encyclopaedia Britannicas, this guy unloaded $117,000.

I also didn’t understand when, visiting Zero Mostel, then starring in Broadway’s decades ago “A Funny Thing Happened on the Way to the Forum,” I saw the walls filled with his homemade sketches, paint splotches, drawings, gaudy outlandish claptrap. Why? What’s bad with elegant wallpaper?

Zero told me: “You buy modern art to cover a hole in the wall, then you find out the hole looks better.”

Abstract art gives new meaning to the word “whaaa?” I figure if I ever had the opportunity to interview Picasso, he’d probably tell me: “The world today doesn’t make sense so why paint pictures that do?”

We’re talking poverty. Greece is in the toilet, Spain’s not doing well, France is failing, America’s not exactly flush, Hungary — forget about it, England has little else but its trifle pudding — and Christie’s just unloaded $495 million in art.

Museums, with such inventory that treasures unseen lie in basement vaults, sue one another for who owns what. Countries, reclaiming longlost sculptures, are suing countries. Paintings Nazis allegedly confiscated have spawned a global industry. Native Americans, claiming their sacred Hopi objects were stolen, are battling legally. Copies, fakes, remakes abound. A fakola Aphrodite went for $425,000.

Art fraud is endemic. Buyers must be careful. One guy, trying to sell “an original Van Gogh,” said it was the only picture Van Gogh ever did in ballpoint.

This I heard at the Guggenheim: “First the good news. Your canvas is an original Palagrini. Finished during the Renaissance. Now the bad news. Palagrini was a plumber.”

Only in New York, kids, only in New York.