I have a store on HipStamp so need to be careful about my opinions.

If you are primarily interested in buying/selling within the US market, then I believe HipStamp is catering well to that small niche, with sales now on a par or better than MR's defunct bidStart site.
On the other hand, I sell mainly stamps from the Pacific area and use SG catalogue numbers, so I feel like I'm swimming against the current there, with just the occasional sale.
All currency is US$, 99% of stamps are described using Scotts, sorting by catalogue favours displaying all Scott numbered stamps first and there is a separate 'Catalog' feature that only supports Scott numbers.
At least SCF knows the meaning of 'International'.
