In early 1980, Argenis Velasquez quit working behind the counter of a street cafe in hopes of landing a higher-paying job. But several months later, unable to find anything, he left the mainstream economy to begin repairing shoes on a busy downtown street corner.

"When I started out, there weren't too many zapateros," said Velasquez, 28, married and a father of four. "Now there are a lot of people repairing shoes. It's not the best job in the world, but it provides enough for me and my family."

Velasquez symbolizes two major trends in Venezuela, where, with the steady drop in oil prices, the economy is expected to decline for the eighth straight year in 1986.

One development is the growth in the informal, or underground, economy following a sharp increase in the official unemployment rate. The other trend is a change in consumer buying habits because of a 35 percent drop in purchasing power since 1979.

Both of these trends have been occurring in most other Latin American countries, which also have been wracked by years of economic decline.

In Venezuela during the 1960s and 1970s, rising oil prices fueled strong growth, which provided manufacturing and service-sector jobs for the tens of thousands who flocked to Caracas every year. A steadily expanding public sector created thousands of other jobs. As late as 1978, the unemployment rate was 3 percent.

But the following year, the economy began to stagnate, and it has been declining ever since as oil prices have dropped. Falling revenue also has forced the government to halt the growth in public employment.

Oil income accounts for 90 percent of export earnings, 60 percent of government revenue and 25 percent of gross national product.

Analysts thought the economy would register positive growth this year, but the crash in oil prices has them forecasting no upturn until 1988.

With the unemployment rate at 13 percent, workers like Velasquez have been unable to find jobs in traditional occupations, and have joined the informal economy, doing everything from peddling fruit from shopping carts in downtown Caracas to selling lottery tickets at bus stations or hawking ice cream at the beach.

The growth in the informal economy has softened the political and social impact of Venezuela's downturn. "With unemployment rising and with no effective welfare net for Venezuela's marginal population, street peddling provides a socially tolerable alternative to begging, robbery or starvation," one analyst wrote recently.

Crime has risen, however, as has the number of beggars.

Edmond Saade, president of a market research company, estimates that the informal economy has grown to 10 percent of economic output. "This kind of activity is growing by leaps and bounds," he said.

Venezuela's informal economy, though, pales in comparison with those in countries such as Peru, Bolivia and Brazil, where it has become institutionalized. Peru's informal economy is said to account for an estimated 40 percent of economic output, the highest figure in Latin America.

With the decline in purchasing power, Venezuelans have changed their buying habits. They are postponing purchases of major items, buying less-expensive products and doing less impulse buying, in a change affecting virtually every industry, from cars to shoes to alcoholic beverages.

"Everyone is trying to make their bolivars stretch farther," said the head of one U.S. multinational company here.

Per-capita demand for virtually every industry except food has fallen. "Consumers are still putting their priority on food products at the expense of other items," Saade said.

Venezuelans are shopping much more carefully than before. "People are planning their purchases more, going to the store with a list of what they want and with a fixed amount of money," Saade said. "They buy only what they need."

Sales of new cars and home appliances have plummeted. "Back in the boom times, people would get a new car every two or three years," said Bill Strange, head of General Motors Corp.'s finance company here. "Now they probably buy a car every five years."

New car and truck sales, which reached 189,000 units in 1978, were 115,000 last year.

Instead of buying a new car when their current vehicle develops an engine knock or a door gets dented, Venezuelans now ignore the problem or have it repaired. "There has been a big increase in business for mechanics," Strange said.

Similarly, instead of buying new shoes when the current pair gets worn, Venezuelans have them repaired by men like Velasquez.

Saade said he believes that the changes in consumer spending will have long-lasting effects. "Austerity has lasted five or six years already and will continue for at least two more years," he said. "Even when Venezuelans have more money, this period will have lasted so long it will have left its mark for years to come."