So designers are always stopping by?
M: Yes, the designers come three times a year. They get "inspired."

So, is the influence from what you sold them to what's on the runways obvious to you?
M: I could tell you stories--and I won't. [Laughs] Sometimes they don't even change a stitch. Not one stitch. And a dress that they would buy for $175 will end up in the pages of Vogue for $6,000 and it's the exact same dress but with fabrics and workmanship that isn't as good, but it has their name on it.

Does that annoy you?
M: It does, but there really isn't anything I can do about it. So it's best just to be grateful for the business.

We hear that Jack had a hand in creating the technique behind stonewashed and acidwashed jeans?
M: In the 70s, Jack literally invented stonewashed jeans. And in the 80s he invented acidwashed jeans.
J: Like all good things, it happened accidentally. There was a chemical reaction—I go, wow. Okay. We start to do it for clientele. Then there was a store owner in upstate New York named Tommy Hilfiger: we advertised what we were doing, and he must have read the papers. He started shipping boxes and boxes of damaged jeans [for us to wash them]. This was back in the days when it was only heavy dungarees, everything was so stiff.

But you didn't patent the process?
M: We were very young.
J: They're certain things you can't patent. It was an accidental chemical reaction that we used for something, and it just did it.

Tell us—what should customers look for in a vintage store, or be wary of?
M: When you walk into the store, notice if there's a smell. If there's an odor of mothballs or mustiness, turn around and walk out.

So, everything in your store has been cleaned? How do you clean your clothes?
M: Jack has a degree in dry-cleaning. We're a perfectly green industry, and nothing goes out on the floor until it's absolutely perfect and clean.

Any favorite celebrity sightings?
M: Bruce Willis and his daughters are the nicest people. His kids are lovely—really down to earth.

You had a huge space in Union Square until about five years ago. Why did you leave?
M: Oh, the Batcave! It was 3 floors, 12,000 square feet. Max Brenner [the chocolate restaurant] is there now. The landlord wanted to raise our rent to a million dollars a year.

Cheap Jack's is located at 303 Fifth Avenue, near 31st Street.

Previously: Five minutes with collectible clothing guru Shannon Hoey of vintage shop New York Vintage.