CARLSBAD, Calif.  Marty Ummel feels she paid too much for her house. So do millions of other people who bought at the peak of the housing boom.

What makes Ms. Ummel different is that she is suing her agent, saying it was all his fault.

Ms. Ummel claims that the agent hid the information that similar homes in the neighborhood were selling for less because he feared she would back out and he would lose his $30,000 commission.

Real estate lawyers and brokers say the case, which goes to trial in the north county division of San Diego Superior Court on Monday, is likely to be the first of many in which regretful or resentful buyers seek redress from the agents who found them a home and arranged its purchase.

“When your house appreciates $100,000 in the first six months, you’re not quite as concerned that maybe the valuation was $25,000 or $50,000 off,” said Clifford Horner of the law firm Horner & Singer. “But when your house goes down, you ask: ‘Who might have led me astray here?’ ”