Translation
and those in mourning as those who are not mourning, and those who are rejoicing as those who are not rejoicing, and those who buy things as if they do not possess them,
Paraphrase
If you are showing proper respect to a deceased loved one by observing the traditional time of mourning, you should not focus on that because it will take your focus away from the present realities, rather you should focus only on being faithful to your Lord. If you have just experienced something that is usually followed by an extended time of celebration, you should not focus on that accomplishment because it will hurt your ability to be prepared for hardships to come, instead you should focus only on being faithful to your Lord. If you just made a major purchase, acquisition, or financial investment, don’t focus on the monetary aspect of things because that takes your focus away from spiritual realities, instead focus on being faithful to your Lord.