Questions sent to sellers don't automatically appear in the box at the bottom of the auction - the seller has the option to put them up there IF they think that the info will help other bidders and maybe avoid them having to answer the same queries multiple times. If you ask a question about something that the seller doesn't want made obvious to the bidders, then he's not going to put it up for all to see.
I agree that in this case it's very misleading (and definitely against eBay rules) for him to put "LGB" in the auction heading - but playing devil's advocate for a moment, I think we need to accept that "LGB" has become pretty much a generic term for G scale to many people - just in the same way that all vacuum cleaners were "Hoovers", at least until Dyson came along...