To 'sell out' of something can mean to sell all of a product so it is no longer available (for sale). If you accuse somebody of 'selling out', it is a way of accusing somebody of reducing their moral or ethical standards, usually to make a profit or for personal or financial gain. If you 'sell out', it can also mean to sell part or all of a business to an individual or another company. This is an informal expression, usually used more in informal contexts.
Additional examples with 'sell out' are:
A promotional e-mail urges recipients to book their tickets to a conference soon: "We recommend you take advantage of the pre-sale to book your tickets in advance. We anticipate the conference will sell out quickly, as it has in previous years." Someone in work warns you about a colleague: "I can't trust Nathan. He sells out his colleagues at the first opportunity to gain advantage and climb the career ladder." A small company talks about its refusal to accept a generous offer: "Although we have received a generous offer, we refused to sell out any part of our company to those who do not share our vision."