Personally, irrespective of how the agreement was done, written or verbal, if you asked someone to market your bike for you then I see no reason why you wouldn’t be responsible for any costs incurred in doing so.
I've sold many bikes in the past, the cost of doing so has always mounted up, I've always tried to get the bike to show room standard before selling so agree many hours are spent valeting it. It also takes time to put together a good advert & then you have the costs of ebay, autotrader & the likes, its easy to see why a business would want a few hundred quid to do this.
Unfortunately trying to sell a sports bike in the Autumn is never ideal, its well known that they sell faster as the warmer drier weather arrives, of course a salesman will always say it’s a popular model & would sell in no time! If he told you it would take a year to sell you probably wouldn’t have left it with him to sell.