Function marketplace::buy_dtokens

pub fn buy_dtokens(
    resource_ids: Vec<Vec<u8>>, 
    ns: Vec<U128>, 
    buyer_account: &Address, 
    payer: &Address
) -> Vec<Vec<Vec<u8>>>

Buy more than one dtoken at a time

resource_ids is array of resource_id which used to mark the only commodity in the chain

ns is array of n which is the number of purchases. the length of resource_ids must be the same with the length of ns.

buyer_account is buyer address, need this address signature