Glenn Kelman, the chief executive of Redfin, the online brokerage firm, said there was a danger in pouring huge sums into buying up homes “without having a clear idea of how you’re going to make money on almost every single home.” If that happens, he said, “you’re just putting the housing markets, the capital markets, at some degree of risk.”

Instant buying is a small part of the market, but it is growing at breakneck speed. Zillow bought fewer than 700 homes in 2018; it expects to be buying 5,000 homes per month in three to five years. Opendoor, the first big iBuyer, bought more than 11,000 homes last year and in the past year has raised more than $1 billion to step up its pace.

The companies typically aim to hold homes for 90 days or less before selling them, typically to an individual buyer. For the eventual owner, little changes about the process.

In Phoenix, instant buying accounts for 6 percent of all real estate transactions, according to Mike DelPrete, an industry analyst. And in a sign of how iBuying is reshaping the housing market, Mr. Kelman’s own company is getting into the game, buying homes in California, Colorado and Texas through a program it calls RedfinNow.

Even traditional brokerage firms like Keller Williams and Realogy, which owns Coldwell Banker, Century 21 and other brands, have announced plans for instant-buying programs. The trend is a threat to the brokers’ business model — but if it is going to happen regardless, they would rather get a piece of the action.

There have always been people who need to sell their homes quickly because of a lost job or a sudden move. But selling fast has come at a price, usually a steep discount. Instant buyers promise a much smaller discount, perhaps shaving only 1 or 2 percent off what a homeowner might get in a conventional sale.