

Crassus, qui illas fortunas morte dimiserit, miserum Cn. Opus est, it is wanted, it is necessary, because a want, as requisite or indispensable for the obtaining of some end or object. Necesse est enim miseros esse eos qui centum milibus annorum ante occiderunt- vel potius omnes quicumque nati sunt for it is necessary that those are miserable who died a hundred thousand years ago or rather, all that have ever been born Miserum esse (verbi causa) M. Ramshorn's Latin Synonyms supports this interpretation:

The way I interpret this, opus esse refers to the need for something in order to accomplish something else ("to become a successful politician, you need a lot of friends"), while necesse esse refers to things you actually can't do without ("living beings need food and water").

That which you do not need, is dear even at a farthing." Moreover, the precepts which are given are of great weight in themselves, whether they be woven into the fabric of song, or condensed into prose proverbs, like the famous Wisdom of Cato, "Buy not what you need, but what you must have. One translation of the Seneca letter you refer to begins to suggest a difference:
