Olark cannot be used by sellers at Ebay or at Etsy for instance. Gagler is for people who sell at other websites like marketplaces and platforms.

So it's different in that Olark can't be used by two sides of a marketplace without using a chat application like Adium and having accounts?

Yes. This can be used without any Adium or other IM accounts. Sellers could choose to use any IM client, if they prefer.