bartering between two parties that really know each other like you and your clients, is pretty much risk-free as far as the airlines being able to nail you.
Best bet is to always have clients provide your their address, phone, mileage acct number, letters of authorization to take to the airport, etc.

The real risk is for people who book award tickets for 'third parties' that they have no affiliation with and those 'third parties' may or may not understand and/or care that they are on a mileage award ticket, and do not have a vested interest in protecting the account holder....usually a brokerage situation.

Working with clients is a solid and safe situation.

I do concur with SanDiego1K about the potential hassle factor of booking an award seat. Again, if you have he strong, trusting relationship with your clients, you should request that they give you their account info, so you can make your own booking, then contact your client, to simply have the award ticket redeemed and issued.

Boy, I wish you were MY decorator here in CLE!!