Police are urging the public to use caution when selling electronics on Craigslist after a man placed an ad on the website for his laptop and cellphone and was robbed at gunpoint last week in Dorchester. The man, a 20-year-old resident of a Boston suburb, was robbed by two men Friday in an apartment on Lydon Way, police said. The victim had gone to meet the men to sell them the items after one of them responded to his ad.