You only need auto_increment on the column used as the primary key in the table. Foreign keys would have the AUTO_INCREMENT (if appropriate) in their reference tables.

So:

CREATE TABLE PropertyOffers (
    OfferID INT(10) AUTO_INCREMENT,
    CONSTRAINT PRIMARY KEY (OfferID),
    ClientID INT(8),
    AgentID INT(8),
    PropertyId INT,  -- or whatever the type
    CONSTRAINT regstufk FOREIGN KEY (ClientID) REFERENCES Clients(ClientID),
    CONSTRAINT regstufk FOREIGN KEY (AgentID) REFERENCES Agents(AgentID),  
    CONSTRAINT propertyoffer FOREIGN KEY (PropertyID) REFERENCES Properties(PropertyID) 
);

You have a foreign key that references PropertyID, but it is not defined in the table. You need a column before you can declare it as a foreign key.