sold at可以是说在哪里卖或者是卖什么价钱,sold for可以是说卖了多久or多少钱or什么方式.
例：This novel is sold at a book store. 这本小说在书店有卖.
It is sold at 0.99$. 它卖0.99美元.
That gift have been sold for eight years.（一般来说这种情况下不用sold for,而用have been selling for,因为要尽量避免被动式）I have been selling this gift for eight years.
This TV is sold for 1000$. 这台电视机卖1000美元
Those cards are sold for cash. 这些卡片要现金付款.