You're at a play when suddenly your eye is drawn to an object on the set and you find yourself thinking how good it would look in your living room.

In general, that would not bode well for a stage performance. But in Kira Obolensky's Cabinet of Wonders: An Impossible History, a Gas & Electric Arts production opening tonight at the Wolf Building, much of what's on stage is also on sale.

Really, not virtually.

An honest-to-goodness tag sale takes place one hour before each performance. Ticketholders who see something they like pay for it and take it home that night.

The play concerns the Carcasses, Leopold and Christina, poised on the edge of eviction from their decaying family mansion, dusty heirlooms and all. Christina, hoping against hope that an estate sale can raise enough to save the house, sorts through family artifacts, revealing specious memories and a shocking secret.

In keeping with the plot, Lisa Jo Epstein, who directs the play and is artistic director of Gas & Electric Arts, engaged an antiques and collectibles dealer to fill the Wolf Building's 5,000-square-foot Arts Underground performance space with the kind of intriguing objects one might come across in an old house whose owners never threw anything away - vintage tablecloths, children's hankies, men's hats, ladies' gloves, Photoplay magazines, paint-by-number ballerinas, and 78-r.p.m. records.

From there it was a short step to plan an actual estate sale for the audience, a preview of the one Christina envisions on stage.

This odd audience-participation strategy may be a first, Epstein said. (Anyone who knows otherwise, she said, should certainly speak up.)

At a preview performance Tuesday night, Chris Nester, a Bala Cynwyd contractor engaged in restorations at the Fairmount home of the 19th-century painter Thomas Eakins, bought a dustpan decorated with Dutch figures, a white Fire-King mug with smiling farm animals, and a pair of bronze aluminum salt and pepper shakers, circa 1950 - all for less than $20.

Sarah Hunt, an artist and craftswoman from Center City, considered two bargain-bin dresses at $5 each. And Joe Rodriguez, also of Center City, couldn't stop smiling.

"I love this," he said. "It takes multitasking to a whole new level: See a play and shop."

The sale is more than a congenial coincidence. It allows the audience to relate to the characters on a visceral level.

"When we pick up an object, it is a touch point that automatically catapults us to the moment we got the object, or the moment we first heard the story behind it," Epstein said.

We may go to tag sales and see piles of bowls, for example, but when one draws our touch there is usually a reason. Objects contain the history of who we are, Epstein said.

"The telling of a story about the object is embellished by what we think we know about the past and about the circumstances of the current moment," she said.

"And the loss of an object is more than the loss of a memory. It is loss of a lifeline."

And so the cocktail stirrers and other Mad Men-esque accoutrements for one's wet bar, the evening gowns and silky lingerie for even later in the evening, the sofas, chairs, and gaudy lamps assembled for the set are more than they seem.

Prices start at 50 cents, and every performance brings new treasures provided by the project's curator, Ruth Worthington.

Worthington has more than three decades of experience in arts management and marketing, as well as a passion for objects whose beauty is in the eye of the beholder. She's a member of four antiques collectives in Adamstown, Pa., and owns a warehouse in an undisclosed location that is stuffed with . . . stuff.

In another effort to link the audience to the action on stage, where a box of chocolates touches off a maelstrom of memory, chocolate tastings are planned after every Wednesday performance of Cabinet of Wonders. The real-life sweets are handmade by John & Kira's, nationally recognized artisans in Northern Liberties.

The preshow sale impressed Sandy Griffith, a graduate student from University City.

"Just the scale of it exceeded my expectation," she said, her eyes darting from one treasure-laden table to another.

Obolensky was pleased. The sale, she said, injects an element of real life, "whatever that is," into the center of the play. "It really adds quite a lot to the experience."