Imagine each US citizen could transfer her citizenship to a foreigner, as long as she found somewhere else to live and wasn’t about to die, and the foreigner was at least as old, and wasn’t a terrorist, ex-con, etc. Benefits:

• Citizenship could be collateral for loans for school, houses, etc. 
• We’ll prefer those who’d pay to come, over those who choose to leave. 
• Undesirable poor folk are especially likely to leave. 
• Retirees could be paid to retire more cheaply abroad. 

Similar to those old rules against selling houses to blacks, we could also add more restrictions on to whom citizenship could be transfered. At least if we were willing to publicly own up to the racism, ethnism, etc. that such restrictions embodied. Now folks like Julian Simon and Gary Becker have proposed selling citizenship before, without much success. But it seems to me better marketing to first focus on giving each person a direct benefit: more freedom to use an asset they already own, citizenship. First, get folks to see that selling citizenships makes as much sense as selling houses or club memberships. Then suggest that letting government add to the pool of citizenships for sale might raise revenue, and maybe help the economy as well.