Martinez's fine start to the season has seen him emerge as a target for LaLiga champions Barca and struggling Premier League giants United.
The 22-year-old Argentina international, whose agent has talked up interest from Barca in the past, has scored seven goals this term.
Asked about rumours that Barca and United are poised to activate Martinez's €111million release clause, Marotta told Sky Sport Italia: "This is all media talk.
"Our players are happy to be with us and a player must express its desire to leave if he wants to go. We don't tend to sell important players, Inter are a very important club.
"Of course, part of this job is to deal with situations as they present themselves."
Martinez, who arrived from Racing Club in 2018-19, has scored five Serie A goals this term and seven across all competitions for Antonio Conte's Inter.
Inter defeated Bologna 2-1 on Saturday to stay within a point of defending champions Juventus after 11 games.