Both options have their avantages :
- main character is "online" more often than alt : so it's easier to speak with you and make a deal
- dedicated alt is safer : if you lose it , just make another one
But as Kandarim said the important part is : if you give your HS , you need to have a Leanto on this character and not a claimstone.
For me I'm using a dedicated-alt , with a leanto inside one of my main town; and I'm mostly running two salem-client at a time.
Concerning silver-first Rubberduck is mostly right , but that will depend on your sale : for very small items it will be easier to get it first. I don't care giving silver first for 100 golden flakes , I care if it's 10 gold bar
Bjebr