If the technology is still usable in an underdeveloped country, the United States should make the effort to sell it in the interest of both parties. While there might not be a huge profit made from this, it's reasonable to get as much use out of everything before sending it to the scrap heap. Sending the old stuff to people that need it would be a great humanitarian action for the country and is always a worthwhile effort.
The United States economy will most certainly suffer by not selling past technology to foreign nations. For better or worse, we now live in a global economy. Profit margins are razor thin, and sustaining many companies further production requires selling beyond our country's borders. However, care must be taken that Technology does not fall into the wrong hands.
If the US sells its technology, anyone can produce goods using that knowledge. That drives down the cost of goods and makes the economy less stable. Although there can be altruistic reasons for selling technology to other nations, it is ultimately in the best interests of the US economy not to do so.
The U.S. Economy will not suffer because of not selling its' past technology, and the final form produced from that technology, to other foreign nations. The US is world renowned for its technology. Technological innovators and entrepreneurs still move to the US. The US continues becoming a major technological hub creating a competitive economy.