STILLWATER, Minn. (AP) _ Seven trailer loads of stamped envelopes - perhaps more than 100 million U.S. stamps - have been seized in what postal authorities suspect could be a $23 million operation to reuse postage stamps.

Authorities probably won't finish unloading the boxes, bundles and piles of envelopes from the home, garage and shed of a 71-year-old stamp collector until Monday, Postal Inspector Howard K. Petschel said Tuesday.

''They're stacked all over the place,'' Petschel said. ''We're even finding them in the walls, in the rafters.'' And, he said, there are two barns a few miles away full of more stamps.

Petschel said he expects the investigation into illegal reuse of postage stamps to turn up more than 100 million stamps which could have represented a loss to the U.S. Postal Service of $23 million.

''It's a nightmare,'' Petschel said. ''It's exceeded my wildest imagination.''

The collector told authorities he had not removed cancellation marks from stamps but simply was looking for stamps that had been skipped over by postal service canceling machines, Petschel said.

Reuse of such stamps is illegal, as is altering the cancellation mark on a stamp, he said.

Investigators suspect that some of the bundles of canceled stamps were being sold to others who may have been cleaning the stamps, then selling them for 50 to 70 percent of their face value, Petschel said.

He said agents served a search warrant Thursday on the man's home near Big Lake Carnelian, east of St. Paul, but charges were not expected to be filed until the search has been completed sometime next week.

The affidavit said the man's name and address appeared in documents seized in Denver last year along with a number of washed stamps. He was identified in the affidavit as the source of 1,500 pounds of envelopes and 2 million stamps seized in July 1985 at the home of a Grantsburg, Wis., woman.

The affidavit says Petschel visited the man three times over the summer and bought used stamps and envelopes with stamps on them.

The affidavit, which is not a criminal charge, says the man explained to Petschel how previously used stamps should be soaked from envelopes for reuse.

The man apparently go the used stamps from a mail redemption company that handles mail for companies running contests, the inspector said.

Petschel said that when the Postal Service two years ago began looking into misuse of used stamps they discovered the scheme had become a big business. Last year, $12 million in contraband postage was seized, he said.

''It turned out bigger than we expected. We're not after the person who might reuse one or two stamps, but some of these people had turned the process into a money-making business,'' Petschel said.

The reused stamps apparently were sold to businesses throughout the country. ''One person would lead to another,'' Petschel said. ''The investigtion is continuing and we are looking at several more people.''