ROCKVILLE, Md. (AP) — Police in Maryland say they’re investigating two robberies where people offered items for sale online and were robbed when they met the supposed buyers in person.

Montgomery County police say in a statement that the unrelated robberies happened in the Wheaton area. Police say one robbery took place July 6 after a man offered a pair of tennis shoes for sale on Facebook’s “Marketplace.” Police say when the man met the alleged buyer, the person produced a handgun and stole the shoes.

Police say the second robbery took place July 8 and that the victim was a woman who had advertised a digital camera on the app “OfferUp.” Police say the woman was assaulted and robbed.

Police are urging residents to use so-called “exchange zones” at police stations for transactions.