With the holiday season in full swing, Los Angeles police are warning San Fernando Valley residents about robbers targeting buyers and sellers who use smartphone apps and other online services to begin their transactions.

The LAPD is urging caution after three recent Valley robberies in which the victims and suspects met via the OfferUp app for smartphones.

Over the past two weeks, authorities said, two unrelated robberies took place with criminals posing as potential buyers and robbing sellers at gunpoint of an iPhone in Encino and expensive jewelry in Reseda, both in the West Valley Division.

A third incident was also reported recently in the Devonshire Division area involving a seller whose purse was stolen, said Lt. Kirk Kelley of the West Valley station.

“Our concern is with these three incidents in a short period of time and the holidays, and people selling stuff to raise cash — and we want to get the word out there,” Kelley said.

The holiday season, in particular, is a “concern” for law enforcement.

“People are trying to sell their stuff to raise cash for the holidays, or trying to buy items,” Kelley said. “An iPhone or something else (purchased from a private party) can be cheaper than retail. So during the holidays, these (types of robberies) could increase.”

Kelley says people should be cautious when using all online services that connect buyers and sellers, including Craigslist.

The LAPD gives buyers and sellers the following tips:

• Meet the potential buyer or seller at a safe, public place like a police station parking lot. If they refuse to meet there, it’s a red flag. “If it’s a criminal, immediately they’ll say, ‘No, no, I don’t want to meet there,’” Kelley explained.

• Do not bring a buyer or seller into your home, as they can now see everything you might have to offer a potential robber or burglar. “For one of our robberies, they met in front of an apartment, which is a very bad idea,” Kelley said.

• Bring a companion along for the sale.

• Use extreme caution when selling highly coveted items like cell phones, jewelry, antiques, collector’s items and rare coins.

• A person can be whomever they want to be online. Criminals will create fake Facebook accounts to appear credible.

In August of this year, according to City News Service, a man from Long Beach was sentenced to life in prison without the possibility of parole for killing Cal State Northridge student Abdullah Abdullatif Alkadi, who had placed an online advertisement about a car he was trying to sell.

Agustin Rosendo Fernandez, now 30, was convicted June 9 of first-degree murder for the stabbing death of Alkadi, a native of Saudi Arabia.

Alkadi listed his 2011 Audi in an online advertisement for $36,500, and he agreed to sell the car to Fernandez for $35,000 after an initial meeting, according to a sentencing memorandum filed by Deputy District Attorney Cynthia Barnes.

“When the defendant went to pick up the Audi, he killed Alkadi so he could keep both the Audi and the money,” the prosecutor wrote. “He was eventually arrested and confessed to the homicide.”

LAPD Lt. Kelley said when people are making purchases, if they can’t make it to a police parking lot, they should go inside a public place like a Starbucks, or a restaurant with security cameras.

Anyone with information on these types of crimes can call Lt. Kelley at 818-374-7610. During non-business hours or weekends, contact 877-527-3247, or anonymously contact the L.A. Regional Crime Stoppers at 800-222-8477 or www.lacrimestoppers.org.