While I'm not saying this is what has happened to you, I had a customer once with exactly the same complaint. Every time I tried to explain that it was a brand new unit, he said "but it has scratches all over the face". I told him there was a protective cover on the face but he kept saying it wasn't there. Fortunately, he was in the same city, so I drove over and looked at the monitor. I peeled back the clean plastic protective cover to reveal a nice clean monitor face. He couldn't believe that he couldn't see it.The 507 units have a little tab so it is easy to pull it off but for some reason the 510's do not and it you don't look very close, it really isn't visible. I have had to use my fingernail or very carefully a paring knife to get the shield to where I could peel it off.
If that isn't your problem, I would call TST directly or the dealer you purchased it from for help as TST doesn't put used parts in any orders I have ever received and I've been a dealer for them for 5 years.
Please do keep us posted as this is an important post. Hugs, Di