REFERENCES to foreign tables

Hi (not sure if the Subject: field is correct, but here goes ..),

I'm trying to build some more complexity into the db that I mentioned
recently.  If in the 'clients' table, I have

client_id int NOT NULL PRIMARY KEY CHECK (client_id > 0)
name varchar(50) NOT NULL
[...]

... and in the 'sales' table, I have ...

sale_id int NOT NULL PRIMARY KEY CHECK (sale_id > 0)
sold_to INT NOT NULL REFERENCES clients (client_id)
[...]

So when a sale is entered, postgreSQL keeps a record of who it was sold
to.  Now what happens if I want to delete the client who bought the
item in question ?  I'm going to have a 'sales' table that references a
buyer who no longer exists (in db terms, naturally).  I wondered aloud
(talking to the wall, sort of) if I could reference clients.client_id
to sales.sale_id, but wouldn't that stop my adding a client who hasn't
yet made a purchase ?

Thanks in advance.

D.