Yes, sounds like a scam. There is no need for you to give access to your blockchain account to anyone if you are working with a broker. There are reputable cryptocurrency platforms that will let you trade without a broker, why would you want one? If an address is watch-only, you cannot use its funds, because you do not have access to them. If you met this person in real life, make a contract, list the terms, have them sign it and specify the wallet address they will use for you. Good luck with your trades.