An item arrived that was missing parts, and the seller is willing to send a replacement.

[this_is_not_a_username]

Should I wait for the replacement to arrive before closing the case?

[momcqueen]

DO NOT CLOSE THE CASE!

It is likely (but not entirely impossible) the replacement will not come.

Never close any case, or let it time out, without the matter being resolved to your complete satisfaction. Promises are empty words until you have what you need in hand.

Is this a seller you know and trust? Local? Domestic? Is it being sent with tracking? How long is it supposed to take to arrive?

When you get something that does not meet the standards of the Item Description, it is better for you as a buyer to return it for a full refund and start over. With cases of Significantly Not as Described, the seller is obligated to pay for return shipping in some capacity. You should lose nothing that way whereas if you wait an eternity for a replacement part that never comes, you will be out the functional item and your money. 

[femmefan1946]

DON'T CLOSE THE CASE!!!

 That can't be emphasized too much.

If the 'replacement' doesn't arrive in 15 days, escalate the Item Not As Described dispute by asking eBay to step in.

The seller will be required to send you a Return Shipping Label.

If he doesn't or when the item show as returned, you will be refunded.

DON'T CLOSE THE CASE!!!