@jogil - It seems to depend on area
In North America - eBay
, Hipstamp, then Delcampe
In Europe - Delcampe or eBay
Overall, Delcampe is bigger (more listings) than Hipstamp/Hippostcard combined. It has also been around longer.
Not sure about other regions, certainly see far more Asian & Australian buyers on Delcampe than Hipstamp.
Interesting question - they have stamps, postcards, comics now - other collectibles could certainly be planned - coins, collectible toys, etc. I'm not sure who they could buy - they're still pretty small...
One feature, or problem depending on your viewpoint, with their model is that their stamp/postcard/comic stores are completely separate. So I wanted to list a postcard/comic it's a separate storefront (same look & feel of course), whereas eBay/Delcampe can support many types of products being listed. It does allow them to theoretically specialize more on the specific product, but I'm not a fan of the approach myself.
PS - Josh Sparrow is in their marketing group - nice guy.