The buyer is lying that he found a defect

[v.v_58]

I have received message from buyer that item have a stain. He attached the photo with stain, but not full photo of item just piece of the photo. When I checked the picture, I found that is different item- Knitted, but my item was made of fabric cotton. 

Tell me please, what shell I do? 

[ruby*tuesday]

You don't have to do anything unless your Buyer actually opens an EBay Returns Case for Item Not As Described. Then you would have to deal with it.

If you offered a 30 days Return Policy it does protect you in the main from unscrupulous Buyers claiming Not As Described to obtain a Refund with you then having to bare the Cost of the Return Postage.