The robbery was reported shortly after 8:25 p.m. Monday near the intersection of Market Street and Van Ness Avenue.
The victim, a 29-year-old woman, had agreed to meet with the man to buy a laptop from him, but when they met he took out a knife and robbed her of the cash she had brought, police said.
The suspect, described only as a black man in his 30s or 40s, fled on foot and had not been found as of this morning, according to police.
The woman was not injured in the robbery.
Anyone with information about the case is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.