Are catwiki a Dutch company? Do they hold the coins like an auction house does or is it like eBay where the owner is selling them? Perhaps people are worried that on a new platform things can go pear shaped, such as you buying the coin and it doesn't make an appearance after paying out X, i'm not saying this is the problem, more thinking about the possibilities of why the bidding is slack.
There's also the big whale that is eBay where people are more used to, breaking into a marketshare like they have will take a lot of time IMO. Just looked at the 1817 Geo III Halfcrown, the bid increment is 5 euros a pop, seems a little higher than the eBay increment at the low end, from 29 euros the next step 34 euros, can't quite remember but doesn't eBay start the 5 increment after £50?