If the barn doors are missing, then the optical port is not protected against dust and dirt.
I would ask the seller to provide the results of a loopback tests for all ports.
Connect:
ADAT1 OUT---->ADAT1 IN
ADAT2 OUT---->ADAT2 IN
ADAT3 OUT---->ADAT3 IN
ADAT4 OUT---->ADAT4 IN
Set clock source to internal.
And then a screenshot of RME driver settings. You should see "sync" on all ports.
Then you can buy TOSLINK dust covers to protect the ports where needed.
https://www.delock.com/produkt/64023/merkmale.html
Two missing/damaged protective covers are a material defect and the price should reflect it accordingly.
Make sure you have a written agreement for a full refund if the loopback test results cannot be reproduced on your end, or if any port shows sync issues after delivery.
As enforcing a refund agreement with a private seller can be difficult in practice, I would focus on used items without such defects.
Ask the seller if he is the original owner and if he still has the purchase receipt. Also ask how the barn doors went missing, a straight answer (or lack of one) tells you a lot.
BR Ramses - HDSPe MADI FX, M-1620 Pro D, 12Mic, UFX III, ADI-2 Pro FS R BE, Nuendo 15, Win10 IoT Ent