I provide an "out" for several less assertive family members, partially because when I was younger and less assertive myself, their tendencies to get corralled by pushy people was a very negative experience for me as well. It's become self-preservation for a number of reasons. So as soon as Mom feels like she "needs to be nice" to the guy in the grocery store parking lot with a wheeled cooler full of meat he's trying to sell her, if I see her getting that apologetic smile on her face that means she'll be writing him a check for $200 for the lot after a 20 minute hard luck story, I very quickly interject with a "Mom, we need to go now," with my hand on her arm. Rarely do I address the salesperson. There is only one time my mother was actually interested in the spiel (and I think it WAS the Dead Sea Salt stuff), and I said, "Fine, meet me at ___." Naturally, she ended up spending a bundle on that crap, too.
I see what people are saying about not treating other adults like children, but one time, I DID have to interrupt when Mom started waffling with a firm, "No, Mom. You can't afford that. We need to go now." Because when she spends $200 on a cooler full of meat that some guy has convinced her to buy because his "refrigerator truck broke down and it's all going to go bad and I will lose my job!" and then doesn't have the money for her bills, guess who gets to pay her bills?