She reports:

Such transactions sometimes take place when sellers want only serious buyers in their houses, in highly sought-after locations, with houses that are architecturally distinctive or for privacy, when agents are representing celebrity buyers or sellers.

But agents say they also occur with properties that have what specific buyers want -- typically, excellent condition, nice appearance, fair price -- with buyers who have told agents that they're ready to purchase a place that meets certain criteria, with motivated sellers and those who hope to sell with less marketing. Such transactions also occur in private deals among neighbors or friends.

It's difficult to track how often this happens. But Metropolitan Regional Information Systems, keeper of the multiple list, notes how many homes sell within 30 days. In February in the Baltimore metro area, it was 286 -- 18 percent of all sales.