The Supreme Court, expanding police authority to crack down on pornographic bookstores, said yesterday that police do not need a warrant if they enter a bookstore and buy, rather than seize, allegedly obscene materials.

The 7-to-2 ruling reinstated the 1981 conviction and $500 fine of Baxter Macon, an employe at Silver News Inc., a bookstore in Hyattsville.

The court, nearing the end of its 1984-85 term, also agreed to hear arguments this fall concerning the federal government's power to require hospitals to provide treatment for infants born with severe birth defects, and two cases involving the degree to which government must bend regulations to accommodate individuals' religious beliefs.

In the obscenity case, a Maryland appeals court last year overturned Macon's conviction. The appeals court said the Prince George's County detectives, who were conducting a countywide crackdown on "adult" bookstores, violated Macon's rights under the First Amendment.

The appeals court ruled that after the detectives purchased two sexually explicit magazines, they should have asked a judge to determine whether the materials were obscene before seeking an arrest warrant. Even though the magazines were purchased, the appeals court said, they were, in effect, seized in violation of the Fourth Amendment.

Justice Sandra Day O'Connor, writing for the majority in Maryland v. Macon, reinstated the conviction, ruling that the detectives' purchase of the magazines did not amount to an illegal "seizure."

"The use of undercover officers is essential to the enforcement of vice laws," O'Connor said in a narrow ruling that focused on the Fourth Amendment issue.

"An undercover officer does not violate the Fourth Amendment merely by accepting an offer to do business that is freely made to the public," O'Connor said, noting that most state courts also have said purchase of such materials does not amount to seizure.

O'Connor did not say whether the warrantless arrest was legal, but said that, even if the arrest was improper, the magazines could still be used as evidence.

Justice William J. Brennan Jr, joined by Justice Thurgood Marshall, dissented. The result of the majority's "endorsement of the government's abuse of the arrest power . . . to enforce norms of taste" would lead to "suppression of . . . nonobscene expression," Brennan said. "These stealthy encroachments upon our liberties sanctioned in the state's present effort to combat vice may become potent weapons in a future effort to shackle political dissenters and stifle their voices."

In another decision yesterday, the justices, acting without the participation of Justice Lewis F. Powell Jr., divided evenly -- and thus issued no opinion -- in a case that had been seen as a potentially important test of what accommodation government must make for the religious beliefs of certain persons.

The tie vote in Jensen v. Quaring upholds a federal appeals court decision that said Nebraska must provide a driver's license to a woman who refused on religious grounds to have her picture taken.

But the tie does not set a nationwide precedent. Rather, it means that the lower court ruling has no effect outside the seven states under the jurisdiction of the 8th U.S. Circuit Court of Appeals, based in St. Louis.

The appeals court ruling, which conflicts with conclusions reached by other federal and state courts issue, said the denial of a license infringed on the woman's constitutional right to exercise her religion freely.

The justices agreed yesterday, however, to hear arguments in the fall in two cases that raise similar issues involving constitutional protections for free exercise of religion. One case, Goldman v. Weinberger, decided last year by the U.S. Court of Appeals for the District of Columbia, involves an appeal by an Orthodox Jewish psychologist in the Air Force who wanted to wear his yarmulke while on duty.

Another case accepted by the justices is Heckler v. Stephen J. Roy. That case involves a man who objects on religious grounds to a requirement that his child have an identification number in order to collect welfare benefits.

The tie votes on eight cases this term equal a record set in 1940 for the most tie votes in a term, according to court records dating back to 1927. The ties are the result of the prolonged absence of Powell, a member of the court's moderate center who was hospitalized earlier this year.

Powell missed 56 cases. Eight have resulted in ties and another five cases were reargued, four this term. The justices yesterday ordered the fifth case, Eastern Air Lines v. Mahfoud, to be reargued next term.

The justices also agreed yesterday to hear the Reagan administration's arguments next fall stemming from the controversial Baby Jane Doe case in New York involving treatment of babies born with severe birth defects.

In Heckler v. American Hospital Association, the administration is arguing that the Rehabilitation Act of 1973, which bans discrimination against the handicapped, gives the federal government broad power to regulate the treatment that such infants receive from hospitals receiving federal funds.

The administration first issued similar regulations in 1982 in response to a case in Bloomington, Ind., in which the parents of a baby with Down's syndrome and other ailments rejected life-saving surgery for the child.

The Health and Human Services Department regulations were designed to force doctors to treat such infants, but the regulations were struck down by a federal court. A new set of regulations was issued in 1984, but those were struck down by a federal judge and later by the 2nd U.S. Circuit Court of Appeals in New York.

In that case, the appeals court said the administration had no authority under the Rehabilitation Act to obtain the records of a baby girl born with spina bifida and an abnormally small head.

Congress, in response to the federal court rulings, passed a law last year that said states could qualify for grants to state child-protection agencies only if they take steps to monitor and investigate the treatment of handicapped newborns.

The administration argues that the law is inadequate because it limits the power of the federal government to an oversight function and does not allow it to intervene directly in specific cases and to force treatment.