The Reagan administration has enlisted the help of the National Association of Realtors in its effort to identify prime pieces of federal real estate, but Interior Secretary James G. Watt reiterated yesterday that the administration is not considering "any massive sell-off" of federal lands.

Watt, testifying at a Senate workshop on the subject, offered yet another estimate of what the administration thinks it can sell. "At this point," he said, "we are considering" selling 5 percent of the more than 700 million acres of the federal lands. "And at this point, that is only a hypothetical figure."

Administration officials have predicted they will raise $17 billion in the next five years by selling "surplus" federal lands, mainly those now managed by the Interior and Agriculture departments. The policy is billed as an effort to rid the government of unneeded properties and to help reduce the deficit.

Known to some as "privatization," the sales plan has been attacked by environmentalists and some western state officials as "piratization"--a bonanza for private interests at taxpayer expense. They cite the warnings of several administration officials who contend the sales program will raise far less than predicted.

Watt said yesterday that the National Association of Realtors has been enlisted to "nominate" federal lands with a high market value to help officials meet the ambitious revenue targets. More than 1,800 local boards of realtors are at work on the search, a spokesman for the group said yesterday.

The administration has kept secret the specific sites under consideration, rattling political nerves in several western states where the federal government owns up to two-thirds of the land. Responding to these growing concerns, Watt stressed yesterday that the government "will not, N-O-T" sell national parks, wildlife refuges, wilderness areas, wild and scenic rivers, Indian trust lands or national recreation areas.

Bruce I. Selfon, acting director of the White House Property Review Board that will coordinate the sales, said at a later workshop that the administration has also decided not to sell lands over coal, geothermal and mineral resources. The board must still consider whether to sell grazing lands and certain federal lands requested for local and state use under Watt's Good Neighbor Policy, Selfon said.

Like many features of the complex sales plan, the number of acres to be put on the block remains sketchy. White House officials announced plans last month to sell 200,000 federal acres of military bases, abandoned buildings and open land in 1983-84. Watt pegged 4 million acres of Interior-owned grazing and scrub desert lands in 10 western states, but said his department will come up with more.