Search for the @ username on the web and see if they spammed porn or other illegal crap in the past.
Ask why the seller is selling it, usually if it's a scam they'll give you a really bad reason they're selling.
Else if the search is clean and there's no mob going after the account, you should be OK. IG will probably verify you if you're on a different region, so if this is legit and long term, prepare to get verified.