Stanley Marcus Jr. thought when he invested $9,000 in a precious-metals company that his retirement in Lusby, Md., would have a silver lining. Instead, he told a House subcommittee yesterday, he lost it all, cheated by the metals company.

Betty Moore and her disabled husband Bayard mortgaged their home and invested $25,000 in a franchise selling pies in McKeesport, Pa. They lost their savings to fraudulent franchisers.

Marcus, Moore and other people, most of them elderly, told a House Postal Service subcommittee yesterday how they and their friends had been cheated by frauds operating through the mails. They testified in support of legislation to strengthen the U.S. Postal Service's efforts against mail fraud by giving it subpoena and other enforcement powers similar to those of the Federal Trade Commission and federal inspectors general.

The legislation would give the postal inspection service the same access to books, records and other documents as inspectors generals now possess.

The legislation also would prevent fraudulent operators from changing their names or moving to other locations to swindle new customers after they have been caught. Currently, the postal service must initiate a new administrative proceeding each time a company moves or changes its name. The proposal would authorize a civil penalty of up to $10,000 a day for such companies.

However, Rep. William D. Ford (D-Mich.) said the postal service could be getting too much authority and could use it to gain access to competitors' records or to censor publications. He also cited concern that the postal service would divert funds from its mail service to police activities with postal employes "playing G-man."

Postal inspectors receive an average of about 200,000 mail fraud complaints annually.