for sale的意思是“待售”“供出售”，即“用于出售”。on sale有两个意思，一是表示“出售”，与for sale不同，它表示的是“进入销售环节”“处于被销售的状态”；二是表示“廉价出售”“打折出售”。
[mass noun]the exchange of a commodity for money; the action of selling something
we withdrew it from sale.
the sale has fallen through.
(sales)a quantity or amount sold.
price cuts failed to boost sales.
(sales)the activity or business of selling products.
director of sales and marketing.
an event for the rapid disposal of goods at reduced prices for a period, especially at the end of a season
a closing-down sale.
[often with modifier]a public or charitable event at which goods are sold.
a public auction.
(up) for sale
offered for purchase; to be bought
cars for sale at reasonable prices.
offered for purchase
the November issue is on sale now.
(N. Amer.)offered for purchase at a reduced price.
sale or return
[usu. as modifier](Brit.)an arrangement by which a purchaser takes a quantity of goods with the right of returning surplus goods without payment
booksellers normally order books on a sale-or-return basis.