Sigma 50mm 1.4 AF issue. Is it worth sending to Sigma?

[anfibil]

Hi guys,


I purchased an used Sigma 50mm 1.4 (Nikon mount) from a very reputable seller on ebay last week and just had a chance to test it this weekend.
I have a D5100 and several other DX lenses, none of which appearing to have any AF issues with this body.


Now for the reason I'm writing this thread...


Although the previous owner informed me he had no issues with the lens (and I have absolutely no reason to think he is being dishonest) while using it with a D300, I found myself extremely annoyed with what I can only assume is some sort of problem with the AF. Many times I would hold the shutter release and wait for the lens to focus, but it would simply never do it. However, when that happened I could hear some noise inside the lens as if the AF motor were trying to move the glass but instead it was stuck somehow. After several attempts and after playing around with the focus ring, the AF would eventually come back to normal, but I don't think I can rely on it. Has anyone ever experienced something like that with this lens or any other?


Also (and this could be me being paranoid but...), I think this copy back focuses most of the times when the AF actually works.


The seller offered to return part of the money I paid for the lens so that I can send it to sigma. I was wondering if anyone would have an idea of how much it would cost for me to have the lens serviced by an authorized repair center and if it really is worth sending it. I hear so many people complaining that they sent this lens for for repair and it came back still back-focusing. I have the blank warranty card but not the original receipt, so I'm guessing I'll have to cover the whole thing with the help of the ebay seller. Can anyone confirm that this is the case when you don't have the original receipt? Would there be a way to use the warranty given that I only have the warranty card?


Thanks!