Well, it is for a "New Speakout" customer only.
That means it is the Phone/Sim being activated for the first time with SO.
"bought this speakout SIM from a person who sold it to me?" – I am assuming this person – was not a store cashier who gave you a new SIM.
If you bought the SIM from another person they have already activated it/used it – so you will not be able to get $10 if you use a referral code from someone else.
But – if you refer someone else then you can get $10 credit.- call customer care at SO and ask for you referral code.
Hope this answers your Query