The church in Acts had people who sold their possessions and gave to those without. Why doesn't the church do that today? Is it a sin to deny someone something because you want to use it?
An example of the selling of the possessions that you referenced in Acts 4:32 - 5:11 there are two different situations. Let's look at both:
"Now the full number of those who believed were of one heart and soul, and no one said that any of the things that belonged to him was his own, but they had everything in common" (Acts 4:32).
We don't have any evidence that the church asked everyone to sell everything they had and give it to the church to distribute evenly. What we do know is: "Thus Joseph, who was also called by the apostles Barnabas (which means son of encouragement), a Levite, a native of Cyprus, sold a field that belonged to him and brought the money and laid it at the apostles' feet" (Acts 4:36-37). This man, Joseph, sold a field. It could have been one field of many that he owned. The lesson we learn from Joseph is that he gave up something of his own in order to help his brothers and sisters in Christ. Certainly a great example for us all.
II. Ananias and Sapphira in Acts 5:1-11.
But Peter said, “Ananias, why has Satan filled your heart to lie to the Holy Spirit and to keep back for yourself part of the proceeds of the land? While it remained unsold, did it not remain your own? And after it was sold, was it not at your disposal? Why is it that you have contrived this deed in your heart? You have not lied to man but to God” (Acts 5:3-4).
Peter was asking them why they lied. It was their own possession, and they were free to do with the funds as they willed. They decided to give a portion of their profit from the land. The problem was that they gave a portion but said they gave the whole sum.
The difference between these two scenarios of giving was not in the act or the means of how the funds were divided but in the difference of attitude. Joseph was simply seeking to help. Ananias and Sapphira were being dishonest perhaps in order to gain recognition (Matthew 6:1).
What does giving look like in our local church today? Each local congregation today should be equipped with elders who through studying the Scriptures and looking after the local group can assist needy saints spiritually and physically. Most congregations choose to use part of the contribution given every first day of the week (I Corinthians 16:1-2) for the purpose of taking care of fellow brothers and sisters in Christ who have monetary needs. While we don't have to literally sell items or land as was sometimes necessary in the first century to be able to give, we can take money from an account that we have personally set aside just for this purpose. This is an organized and commanded method of giving. I have known of situations where families have had tragic life events that caused major strains financially and their fellow brothers and sisters took up a separate collection just for that one family or group of individuals.
The answer to your second questions falls more in line with our attitude. In our world today many struggle with the differences between needs and wants. Jesus focused his teachings not solely on the actions of man but on the attitude or motivation behind it.
“You have heard that it was said, ‘You shall not commit adultery.’ But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart" (Matthew 5:27-28).
The following verses can help you make the decision in your own heart.
"If a brother or sister is poorly clothed and lacking in daily food, and one of you says to them, “Go in peace, be warmed and filled,” without giving them the things needed for the body, what good is that? So also faith by itself, if it does not have works, is dead" (James 2:16-17).
“Then he will say to those on his left, ‘Depart from me, you cursed, into the eternal fire prepared for the devil and his angels. For I was hungry and you gave me no food, I was thirsty and you gave me no drink, I was a stranger and you did not welcome me, naked and you did not clothe me, sick and in prison and you did not visit me.’ Then they also will answer, saying, ‘Lord, when did we see you hungry or thirsty or a stranger or naked or sick or in prison, and did not minister to you?’ Then he will answer them, saying, ‘Truly, I say to you, as you did not do it to one of the least of these, you did not do it to me.’ And these will go away into eternal punishment, but the righteous into eternal life" (Matthew 25:41-46).