On Valentine's Day, the first of about 600 hopefuls began to line up for a three-day vigil outside the sales tent of a Boynton Beach, Fla., developer, to try to nail a condo.

Among them were a couple of stand-ins for real-estate agent Peter Celnicker, whose investor clients were clamoring for units.

Celnicker was elated when they snagged two — priced at $390,000 and $465,000 — and plopped down two $15,000 certified checks to reserve the unbuilt units.

"Fabulous deal," the Delray Beach, Fla., agent pronounced. "Fabulous."

Two days later, a woman who had not fared so fabulously offered Celnicker's clients $50,000 for each reservation at the sold-out development. Celnicker said they turned her down flat, expecting to flip their deals for sweeter profits down the road.

Flipping is the practice of buying properties to quickly sell them, an investment strategy that has become wildly popular across the country. Disappointed with the stock market and dazzled by double-digit property-appreciation rates, amateur investors — apparently of every income stripe — are investing in real estate in droves.

They are snapping up everything from condo conversions in Chicago suburbs to new three-bedroom ranch homes in the Arizona desert.

Nationwide, ordinary people — armed with bargain mortgages, pooled family savings and cashed-out home equity — are investing at levels that are starting to worry economic analysts.

Their numbers are hard to track. But by one count, investors bought nearly 8.5 percent of all homes sold in last year's record U.S. market, according to Loan Performance, a California housing-data firm.

In 2000, the company estimated investor buyers were 5.8 percent of the market.

Their effects on rising home prices, not to mention what might happen in a real-estate "bubble," are being noticed.

In December, Federal Reserve officials warned they were concerned about speculative demand affecting housing prices and said they were watching some regions' activity closely.

Last year, speculators went into overdrive in Las Vegas, Phoenix, many parts of California and southern Florida, buying perhaps 15 percent or more of homes sold. In those areas, people who were buying a home as a residence complained loudly that they were being pushed out of the market.

Besides driving up housing prices and assessments, a flood of investor-owned houses can depress prices when a market loses its sizzle.

If price-appreciation rates slow — or worse, decline — speculators have more freedom to cut their asking price, sell quickly and get out. Left behind are occupant homeowners, who often must realize a certain profit to buy their next house.

In Phoenix, homebuilders have announced speculators are no longer welcome.

"We were seeing other markets where there were tremendous influxes of investors, particularly Las Vegas," said Susan Williams, a spokeswoman for Element Homes, a Phoenix homebuilder. "They'd come in and buy 10 or 20 homes and flip them or rent them out.

"In some subdivisions, up to 60 percent of the owners were investors, which was leaving the regular homeowners in an uncomfortable situation. The market became flooded with rentals."

When Phoenix began to become a magnet for out-of-state investors, Williams' company and other builders in Phoenix, as well as several in California, started requiring buyers to promise the homes would be their primary or secondary residences for at least a year.

In South Florida, such restrictions generally don't exist.

"We're seeing three times as many buyers as there are units," said Jack McCabe, a Deerfield Beach, Fla., development consultant, who says exuberant investors create artificial demand. "Three-hundred- and 400-unit buildings sell out in a weekend.

"In my very, very conservative estimate, 40 percent of these sales are speculator-driven, and in some projects, it's as high as 80 percent."

Today, some developers court investor interest, said Gail Liss-ner of Appraisal Research Counselors, which gathers data on residential construction.

At least one shuns them.

"Speculators are also our competition [when they resell], while I still may be trying to sell in my building," said Alan Lev, president of the Belgravia Group, a Chicago-area developer whose sales contracts require buyers to live in its buildings.

"Besides, our construction lenders are starting to look at [investor buyers] and say those are not good sales," he said.

Overall, the speculation picture is muddy because there is no single definition of "investor."

For example, vacation homes are investments in some databases, not others. Parents who buy condos to house children in college may not be trackable. Many investments are bought by landlords for the long term.

Or buyers are consumers who see bricks and mortar as an alternative to stock-market nosedives and wimpy returns on bank CDs.