Someone told the cops I sold them weed. They got a search warrant to search my house. How is it legal?

First of all, I assume you did sell them weed since you did not mention that you were innocent. Second, how else would the police get such information. The person was probably arrested and instead of remaining silent he was foolish enough to not only confess but to turn you in to get a break. That is why they eventually catch most people ho deal drugs from their home. The police apply for a warrant and take the person to a judge and the judge evaluates their story to see it it is accurate, recent, trustworthy, and believable. Then he issues a search warrant and the police search your house. Perhaps if you were aware of all of this you would just smoke weed and not decide to become a drug dealer, especially in view of the Draconian penalties for selling drugs in this country, even pot. I do not agree and feel pot should be legal, but no one is fighting and protesting strong enough.

On a search warrant if they can show why they think the person is reliable then yes they can get a warrant to search your house.

If there was probable cause to support the issuance of the search warrant, it is legal. If not, it is an improper search. What county?

Hire a lawyer to review the search warrant. As you stated it, there is insufficient cause to get a warrant. However, they could have set up a controlled buy.

Police need probable cause for a warrant. But the word of the person may not be good enough. I would have to know more about the person such as what is your relationship to the person, what information did he have that led him to believe that you possessed drugs, and did he give a specific description of where the drugs would be found? If the police found drugs, an attorney could file a Motion to Suppress the Evidence.

Yes they can. Someone's words gives rise to probable cause for a search warrant, which the Judge signed.

Yes, all they need is "probable cause" and the statement of another is generally sufficient. Get a criminal defense attorney now.

They are allowed to use a confidential informant.