1)
What is "Segur"?
1.
Rashi: It is Zahav Segur - quality gold. When it is opened (sold), all stores selling [Stam] gold are Segurim (closed - no one will buy other gold).
2)
Why did he add "v'Lo Yishakel Kesef Mechirah"?
1.
Malbim: After saying that it is not acquired via Chalipin (trading gold for it), he teaches that it is not bought for money.
