Id call Ebay and ask them to investigate to confirm whether it was a glitch or not.

I MIGHT also contact the last seller in the buyers feedback and ask a general question re: if they have had any "free postage" glitches. Dont mention the buyers name.

Because, I could photoshop that "free postage" on a mobile or PC screenshot in literally about 5 seconds. *Extremely* easy to do. You dont even need photoshop, it can be done with MSPaint. A photo is not evidence of anything.