to sell all of the supply that you have of something

We sold out of the T-shirts in the first couple of hours. 我们在头几个小时就把T恤衫卖完了。

If a supply of something sells out, there is no more of that thing to buy.

The first issue of the magazine sold out within two days. 杂志的第一期两天内销售一空。

When a film, concert, etc. is sold out, all of the tickets for it have been sold.

We couldn't get seats - the concert was sold out. 我们弄不到座位——音乐会的票卖完了。

to sell your business or part of your business

They decided to sell out to their competitors. 他们决定把公司卖给竞争对手。

to not do what you have promised someone you will do or what you should do because you will get more advantages for yourself if you do something else

French farmers feel they've been sold out by their government in the negotiations. 法国农场主感到在谈判中他们被政府出卖了。

They've sold out to the oil lobby (= done what these people wanted). 他们让自己被石油行业游说团收买了。