The Philadelphia Police Department is out with a warning if you use social media shopping sites. Several people who did got hurt in the process.

Police said a man traveled from New Jersey to North Philadelphia around 1 p.m. on Thanksgiving. Detectives said he had shopped on Facebook Marketplace for HVAC parts. Instead, police said it was a setup.

"Once the victim was led to that grassy lot, two other offenders appeared, one of which was armed with a handgun," Capt. Jason Smith said. "In four of six incidents, our victims were pistol-whipped by one of the offenders."

Smith said his detectives thought the Thanksgiving robbery was similar to other cases.

"We came to the realization that there was a pattern involving these Facebook Marketplace setup robberies," Smith said.

Police said at least four of their victims were duped by low-ball offers on newer cars.

"The offenders were listing vehicles, which were ridiculously low," Smith said.

Some ended up in the hospital after police said the robbers beat them. They're physically OK.

Detectives said the first happened near 26th and Cumberland streets on Oct. 3. The most recent was two weeks ago.

On Friday morning, police took four people into custody who they said are connected with the alleged robberies. Police said they recovered seven firearms.

If you're using social media shopping sites, make safety a priority.

"I highly recommend that they use a lobby of a police district for these types of transactions," Smith said.

In a statement from Facebook parent company META, they recommend always meeting in a public space, sharing your plan with family and friends, saving conversations in the Messenger app, checking your seller's profiles and reviews and reporting suspicious behavior or listings.

The strongest advice from law enforcement:

"If a deal seems too good to be true, it more than likely is," Smith said.

Detectives said if you have any information on these robberies or possible robberies they may not be aware of, call 215-686-TIPS.