IN the three decades since David Dewey of Minneapolis began collecting Chinese antiquities he has donated dozens to favored museums, enriching the Institute of Arts in his hometown as well as Middlebury College in Vermont, where he studied Mandarin.

But his giving days are largely over, he said, pre-empted by guidelines that most museums now follow on what objects they can accept.

“They just won’t take them — can’t take them,” Mr. Dewey said.

Alan M. Dershowitz, the Harvard law professor, is in a similar bind. An antiquities collector, he is eager to sell an Egyptian sarcophagus he bought from Sotheby’s in the early 1990s. But he is stymied, he said, because auction houses are applying tighter policies to the items they accept for consignment.

“I can’t get proof of when it came out of Egypt,” Mr. Dershowitz said.

Across the country measures taken to curb the trade in looted artifacts are making it more difficult for collectors of antiquities to donate, or sell, the cultural treasures that fill their homes, display cases and storage units.