This is a pretty common pushback on marketplaces in general, but in my experience working on several, this only happens in specific situations. I doubt it would be much of a problem for Art in Res, namely because:
1. With higher price points, the security and financial services of the platform become more valuable. Installment payments, for instance, would likely be beyond the abilities of many artists to orchestrate. The platform also likely provides some guarantee on the buyer side that all that money won't go missing and that the product will be delivered as expected.
2. The platform will usually have some language in their agreement that makes off-platform contracts with people you discovered in-platform punishable. If you've ever used a hiring platform like Hired or A-List, you'll know that these fees are persuasive.
With cheaper platforms like UpWork, I'm sure this kind of thing is common, but on more expensive market places it isn't as pervasive as one might think.
Do you charge a fixed fee or a %?