Is there protection against being conned?

Hi all

How do we protect autistic adults against being conned? A relative sold a mobile phone in  phone shop recently.  it cost around  £650 and he swapped it for one that cost aboit £350. When we confronted the shopkeeper he said the relative seemed competent to him and as he has sold the original phone he can't and won't do anything about it.

How do we make sure something like this doesnt happen again? 

thanks in advance