你的两个句子的区别是：一错一对。即：

误：He sold his piano at 260 dollars.

正：He sold the house for a lot of money.

sell sth 后接介词at与for的区别是：at表示售价，for表示金额。比较：

He sold his old car at a low price. 他低价卖掉了他的旧汽车。

Eggs sell at sixty cents a dozen. 鸡蛋售价为每打6毛。

He sold his old car for 800 dollars. 他的旧汽车卖了800美元。