The difference is software on Google's store has infinite leverage and takes almost 0 to scale sales. Every piece of art takes lots of work and cannot scale in the same way. Art works do not scale like software.
With this cut, I feel your platform is taking advantage of artists (they do not know where to sell) rather than helping them.
Even auction commission is only between 12 & 25%.
A good gallery--emphasis on good--does a lot more then simply sell art to the highest bidder. They strategically place artists into 'important' collections, work with museum curators to bring the artists into a more critical narrative, get the artist into group shows and fairs internationally to contextualize the artist in a current scene or trend, collude with art critiques and magazines, and generally help to promote the artist's career over the long term.
All of this sounds somewhat silly outside the art world, but you have to remember that this is a very particular industry based around historicizing high-brow cultural production. When a good collector buys a painting, they are doing more then just buying a physical object they like. They are throwing their own clout behind the artist and saying "I think what this person is doing is important and I stand behind it."
All of this is done with the intention of increasing the profile of the artist which benefits--each in their own way--gallery, the artist, and the collector over the long term.
As far as the sales split between gallery and artist, standard split is roughly 50% with some variance around material expenses and whatnot.
Note, everything I am saying is the sort of ideal story and there are a lot of bad actors in the business. In reality I find the art world rather gross and the premises it is built upon to be deeply flawed.
Auctions take a small cut because they are resellers of art. The artist doesn't get paid when a work sells at auction, the previous owner does.
Art is usually sold by galleries, and the standard cut there is 50%. On top of that, artist and gallery often share discounts, so a work that is sold for a 20% discount means the artist will only see 40% of the total price. For artists, this will be a pleasant surprise rather than a steep expense.
While the customer facing portion of the website appears like a more traditional marketplace, our artist facing website contains tools for artists, and we do more than just list their art, we help them get better at selling, help them promote and market their art, and we provide support wherever needed.
We are artist centric first and foremost, and always heavily consider and consult our artists point of view.
I hear you that, when looked at in a certain light, it might seems high – but we work closely with our artists and they seem to all like that our incentives are aligned. Plus, we do our best to use that 30% in ways that benefit them, e.g. by guaranteeing that they get paid if someone absconds with their work without paying it all the way off.