Need a thumb tack? Well, you could go to the store and buy a package of 50 new, razor-sharp push pins for a buck. Or you could call Geoff Lupo and get a somewhat dulled one for 35 cents. Of course, that includes delivery. And for another 35 cents, he'll throw in a used pen cap. Is this guy for real? That depends on your definition of real. No, Geoff Lupo's thumb tacks are no threat to Staples -- but Lupo may just be one of New York's great underappreciated artistic geniuses. His "Thumb Tack for Sale" posters, and others like it that are hanging on lampposts all over town, are more than just an offering of goods, but a satire of our mundane business culture and a lampoon of the myth that the customer is always right. But you don't have to believe me. "These posters are a serious work of art and a particularly urban one," said Tom Hoving, former director of the Metropolitan Museum of Art and author of "Art for Dummies." "They are a statement on commerce. And the drawings are very reminiscent of Warhol's shoe series." This masterful series of posters began last summer with "Piece of Wood, $5," a critical success albeit a commercial flop. Although the posters were hanging for months, Lupo did not even receive one call. "I realize now that it was too expensive," he said. Next came Lupo's breakthrough work, "Pencil Sharpener, $2." Two dollars might seem like a lot for a used plastic pencil sharpener, but not when you're buying the empty promise of consumer satisfaction. "You really can't put a price on that," said Lupo. One would-be customer did, haggling Lupo down to 75 cents, yet handing over only a quarter and a Band-Aid when Lupo delivered the sharpener. "I had a cut, so I took the deal," Lupo explained. "And then it turned out to be one of those lousy, tiny Band-Aids. I felt cheated. And then I felt that if I felt cheated, that's part of the whole project, too." Lupo began to hit his stride earlier this year with the exhilarating "Nail Clipper...Hardly used!" Critics complained of Lupo's overzealous use of the exclamation mark, but hygienically needy folk responded. "In reality, it was `very used,'" he said. "I ended up buying a new one for $1, so I guess I lost 50 cents on the deal." Next, Lupo offered "Used Marker for Sale. Completely out of ink. 45 cents." Critics again quibbled with the use of the world "completely," claiming it lacked the hit-over-the-head subtlety of his more famous works, but it remains a classic of the genre. A couple of artistic failures -- "Cardboard box. 6" deep. 75 cents" was prurient and infantile while "Plastic Doughboy Heads. $2." was a sell-out -- were followed by what most critics consider Lupo's tours de force: "Pen Cap for Sale" and "Thumb Tack for Sale." "Pen caps are still my biggest seller," Lupo told me. "Everyone's first concern is that it's been chewed. It hasn't. I do try to keep the customer satisfied." That's no problem, considering that Lupo haggles about as well as a dyslexic used car dealer. When one man refused to pay 35 cents, Lupo settled for 10 (and then spent $3 in subway fare to deliver the goods). And a thumb tack purchaser gave him only 10 cents after comparing the sharpness of Lupo's thumb tack against a freshly purchased one. But that's part of the work of art, too. "The whole thing is like a game of tennis: Someone served, so I had to hit the ball back," said Lawrence Abrahamson, who saw Lupo's "Pen Cap for Sale" and bought one (and paid full price -- fool!). "I suggested that he must have found MY pen cap, but when he assured me that it wasn't, I bought one." Abrahamson says he looks forward to more transactions. "Maybe I'll buy a nail clipper," he said. "I do have problems with my nails." --30-- email: gershny@yahoo.com