We are commanded to be generous--not impoverished.

An Illogical Question

Your assumption is quite illogical. "Since it's hard for the rich to enter the kingdom of God, but it's not impossible since nothing is impossible with God, how can any Christian ever buy anything that benefits only themselves? That doesn't even make sense.

Jesus never said "Don't buy anything unless it benefits someone else." I bought groceries over the weekend, and I ate some of them this morning. In your definition, this would somehow be wrong.

An Example from Jesus' Life

Your question assumes that the highest good that a Christian can do is to meet the physical needs of other people. While this is very honorable and all of us ought to do that, we were not created for that purpose, but for another.

In Mark 14, a woman anoints Jesus with a very expensive perfume:

And while he was at Bethany in the house of Simon the leper, as he was reclining at table, a woman came with an alabaster flask of ointment of pure nard, very costly, and she broke the flask and poured it over his head. Mark 14:3 ESV

Judas the traitor is the one who is indignant about the "waste", since the perfume could have been sold and given to the poor (or used to buy a pallet of food for the local food bank). Jesus rebukes Judas, honoring her for her act of worship and sacrifice.

Generosity--not Poverty

For us too, there are greater things for us to do than to give to the poor. Worshiping God and actively seeking to meet the spiritual needs of a world who does not God is much more significant than merely meeting the temporal needs of people.

We are, indeed, commanded to be generous:

As for the rich in this present age, charge them not to be haughty, nor to set their hopes on the uncertainty of riches, but on God, who richly provides us with everything to enjoy. They are to do good, to be rich in good works, to be generous and ready to share, 19 thus storing up treasure for themselves as a good foundation for the future, so that they may take hold of that which is truly life. 1 Timothy 6:17-19 ESV

Paul does not say, "Command those who are rich to stop being rich." He actually says that they ought to be generous, but also that God provides us with everything to enjoy. He also says not to be their hope in riches. Those who are wealthy should keep a right perspective on worldly goods and be mindful that joy and peace come from God and not wealth.

Conclusion

So, yes, let us all give to those who are actually in need, despite their hard work and responsible living. Let us be very generous towards them. Yet, let us not merely meet the physical needs of the world and ignore their spiritual needs. What nobility can be found in extending a person's life here without also providing them with the knowledge of how to have eternal life? Life on earth is quite short and uncertain, while the next life is forever.

We are commanded to be generous--not impoverished.

The Question

Your assumption seems tenuous. "Since it's hard for the rich to enter the kingdom of God, but it's not impossible since nothing is impossible with God, how can any Christian ever buy anything that benefits only themselves? Jesus never said "Don't buy anything unless it benefits someone else by meeting their physical needs."

An Example from Jesus' Life

Your question assumes that the highest good that a Christian can do is to meet the physical needs of other people. While this is very honorable and all of us ought to do that, we were not created for that purpose, but for another.

In Mark 14, a woman anoints Jesus with a very expensive perfume:

And while he was at Bethany in the house of Simon the leper, as he was reclining at table, a woman came with an alabaster flask of ointment of pure nard, very costly, and she broke the flask and poured it over his head. Mark 14:3 ESV

Judas the traitor is the one who is indignant about the "waste", since the perfume could have been sold and given to the poor (or used to buy a pallet of food for the local food bank). Jesus rebukes Judas, honoring her for her act of worship and sacrifice.

Generosity--not Poverty

For us too, there are greater things for us to do than to give to the poor. Worshiping God and actively seeking to meet the spiritual needs of a world who does not God is much more significant than merely meeting the temporal needs of people.

We are, indeed, commanded to be generous:

As for the rich in this present age, charge them not to be haughty, nor to set their hopes on the uncertainty of riches, but on God, who richly provides us with everything to enjoy. They are to do good, to be rich in good works, to be generous and ready to share, 19 thus storing up treasure for themselves as a good foundation for the future, so that they may take hold of that which is truly life. 1 Timothy 6:17-19 ESV

Paul does not say, "Command those who are rich to stop being rich." He actually says that they ought to be generous, but also that God provides us with everything to enjoy. He also says not to be their hope in riches. Those who are wealthy should keep a right perspective on worldly goods and be mindful that joy and peace come from God and not wealth.

Conclusion

So, yes, let us all give to those who are actually in need, despite their hard work and responsible living. Let us be very generous towards them. Yet, let us not merely meet the physical needs of the world and ignore their spiritual needs. What nobility can be found in extending a person's life here without also providing them with the knowledge of how to have eternal life? Life on earth is quite short and uncertain, while the next life is forever.