How do i trade safely?

Hi i’ve recently got back into pokeom go and have some old stuff on my account i was trying too trade with people and they keep offering too fly yo me and i dont know what that means or if i should be worried about trading with people who just fly to me will i get flagged even though all im doing is trading pokemon i got completely legitimately.
( keeps telling me to check rule 3 but i dont understand if them flying is cheating and thats my worry and question hope this is allowed thank you for your time)