Isaac had 2 sons. Esau was born first and had the birthright but didn't care much about it. Jacob desperately wanted the birthright and eventually was successful in cheating his brother out of both the birthright and its associated blessing. While it is true that Jacob deceived his father to steal the blessing, from Esau's point of view he lost it because he sold it (Heb. 12:16).
Jacob had 12 sons but the birthright belonged to Reuben, his firstborn. However, one day Reuben lay with his father's concubine and so was defiled and lost the birthright (1 Chron. 5:1-2). When Jacob blessed his sons he divided the birthright among 3 of Reuben's brothers. Joseph got the double portion of the land, Judah got the kingship, and Levi got the priesthood.
From all this we can see something of what it means to keep the birthright or to sell it. Esau lost the birthright because he despised it, Reuben because he defiled himself. Jacob gained the birthright because he treasured it. (The Lord dealt with him severely for cheating to get it. He was exiled from home, never saw his mother again, and was a virtual slave of his uncle for 20 years.) Joseph got the best blessing of the birthright because he kept himself pure.
In the New Testament the birthright refers to the blessings of the Lord's reward in the coming millennial kingdom. The double portion of the land is the double portion of the enjoyment of Christ. Our best enjoyment of Christ today is not worthy of comparison with the joy of the bride and bridegroom in that day. The overcoming believers will also be kings and priests with the Lord in His kingdom. As kings, they will represent the Lord to rule over the earth with His authority. As priests, they will bring the Lord to people and bring people to the Lord. All these blessings are our birthright as long as we don't sell it because we believers are the firstborn ones in Christ (Heb. 12:23). But if we are unable or unwilling to rule over our appetites and lusts today, how can we expect to rule with the Lord then?