Tie-in arrangements: is an agreement between a buyer and seller that as a condition of the sale they must purchase additional services or products from them.
i.e. A real estate developer/owner says in order to purchase their property you must use their referred mortgage company, title company or attorney. Or you can't use a buyer's agent, although you are compensating them yourself.

This is part of the "One Stop Shop" scams. In many cases everyone is working together.
The reason this condition is put in place is because in many cases a reputable business wouldn't allow the seller/Scammer to get away with certain fraudalent practices under their eye.
Solution: Research your professionals and hire people that come highly recommended and you trust. Also try to hire professionals who came from different referrals sources and possibly don't know each other, that way everyone is working in your best interest and not together for their own.