I wouldn't give the sellers name on the internet. It is unwise to give details such as names, cities, states, des names. Anything can be found on the internet and it would be dangerous to the family member, despite what they are doing.
The best option is to contact the prescribers office, or local police dept, and let them know this person is selling their meds. Both can direct them to get help.
You would have to call her doctor, and discuss it with them. You can anonymously report the misuse/selling of the meds.