Again as often happen, Internet forum 1% users against 99% of non "nerdy" cyclists.
If i asked you question about your computer serial number or say motherboard type or something similar that you might be fairly knowlegeable but not nerdy about, could you provide the information someone asked for? if not does it make it ropey or stolen? No it means the person does not know how to find or aquire the information you require.
Again reciepts are easy lost and not every one even know what bike shepard is.
My advice would be like buying anything second hand worth a few quid, have a list of questions, if the deal sounds to good to be true it probably is. If your not happy with the answers walk away.