Used to be a fella could pick up a six of Billy Beer and a bag of pretzels for less than four bucks. So what if people said the brew tasted like transmission fluid? A trend's a trend. If they came out with "Nancy Champale" or "Ronnie's Rum" next week you'd have to try it.

Billy Beer, which was bumped from the shelves after only a few months after it came out in December 1977, is now back: An ad in Los Angeles is offering a six-pack for $7,-500 (that's without pretzels) and people in the Washington area have bought individual cans for anywhere from $60 to $600. The classified sections of The Post and Star are filled with ads offering Billy Beer at Dom Perignon-plus prices.

"God almighty! And I don't have none of it here!" a shocked Billy Carter told Newsweek a few weeks ago. Indeed. Carter just might have been able to avoid auctioning off his Georgia house and gas station last month if he had filled his garage with Billy Beer.

Dusty Cassidy of Washington says he sold four cans of $400 ech. "I'm pretty shocked," said Cassidy. "When I first bought the beer it was $2 for a six-pack. It was something new and I got it as soon as it hit the racks. I drank one can and threw it in the trash. After that, I just saved the rest."

Still, the sellers with the most moxie are finding the market for Billy Beer erratic.

"I'm asking $5,000 [for a six-pack] but so far I'm not really getting any nibbles," says David Swanson of Stockton, Calif. "On 'The Merv Griffin Show' about three weeks ago some lady said that cans were going for $600 apiece. I've heard of it selling for $1,000 a can, but that sounds like hearsay to me . . . It's gotten to the point where too many people are selling for too much."

According to Bob kelly, general sales manager for F.X. Matts Brewery of Utica, N.Y., one of the brewers of Billy Beer, Carter selected the beer's distinctive flavor under trying circumstances. "I met Billy when he was up here in Utica for a snowmobile championship," says Kelly. "I said to to him, 'Frankly, Billy, I really don't like the taste of this beer.' He said it was okay if you drank about six or seven of them. So I asked him how he came up with that taste and he said, 'Frankly, I was drunk when I picked it.' That's the kind of guy he is." If Carter had been on the wagon at the time -- as he is now -- he couldn't have found a worse flavor.

But even if most agree that Billy Beer was never in danger of surpassing the great grogs of Amsterdam and Munich, no one really knows what started off the seller explosion. Nevertheless, it seems as though anyone who saved Billy Carter's cans is dusting them off and putting them up for sale.

"As a matterr of fact, I've got a six-pack signed by Billy," says Kelly. "Of course, he drank half of them himself when I met him. If the Cans are worth so much, maybe I ought to sell them."

"I have a large collection of beer cans including Billy Beer, several Cone Tops and a bunch of air cans," says Mike Colabucci, 14, of Kensington, Md. "I had 18 Billy Beers but now I'm down to two. I've made anywhere from $60 to $120 each. The going rate is $100 a can but people that don't know what they're doing can get sucked into spending a lot more."

Colabucci, who has also sold a 1957 Willie Mays baseball card for $700 and a first-edition Spiderman comic book for $300, says he isn't sure what will happen to the Billy Beer market in the future. "It's like stocks," he says. "They can go up or down. I got rid of mine now because my parents are building a bedroom in the basement where I keep my collection."

Colabucci says there are nuances of the Billy Beer biz. Full is better than empty. Cans opened from the bottom are better than ones opened from the top. And one would no sooner trade a Utica-brewed goldtop can for a Louisville-brewed silver-top than trade a Willie Mays for a Joe Pepitone.

A final word on Billy Beer: Sell it or buy it. But, as Billy himself says, "I should have held onto the bastards instead of trying to drink the sorry stuff.