Some people have things to hide. If you sound like you know what you are talking about then they will start to think you might see whatever it is that isn't quite right. Something like a crack they are trying to hide, or a bulge in a tube from a prang.
Also could be that the seller is just lazy, or weird . I bought something off ebay recently from a pretty strange character. I asked a couple of questions, to which i got more info and pics than i asked for, but to a third i got no reply. It was all friendly and cheery between us.
After a couple of days I thought i would buy it anyway, but that is when i found out i had been blocked ,as a buyer, when i hit the BiN button. No replies to my messages asking what was going on so i may have also had my messages blocked and not just as a buyer.
How odd i thought so I created a brand new account and bought the item as a user with zero purchases, or feedback. Strange to block someone because they ask questions, as that must be too much hassle, yet will sell to someone who has no history and who could be a right pain in the @rse.