The Secretary of Agriculture is authorized, subject to such conditions as he may prescribe, to sell forest-tree seed and nursery stock to States and political subdivisions thereof and to public agencies of other countries, at rates not less than the actual or estimated cost to the United States of procuring or producing such seed or nursery stock, moneys received from the sale thereof to be credited to the appropriation or appropriations of the Forest Service currently available for the procurement or production of seed or nursery stock at the time such moneys are deposited: Provided, That the Secretary of Agriculture may exchange with such public agencies forest-tree seed and nursery stock for forest-tree seed or nursery stock of the same or different species upon a determination that such exchange is in the interest of the United States and that the value of the property given in exchange does not exceed the value of the property received: Provided further, That no nursery stock shall be sold or exchanged under this section as ornamental or other stock for landscape planting of the types commonly grown by established commercial nurserymen.