Yeah, a number of places have tried some variation on this. The problem is that the intermediaries often end up looking (or actually beings) somewhat shady, which shakes customer trust: people don't want to spend their money there. At least with Dlsite it all feels a bit more polished and professional.
Also, Foxy! Not to call you out, but are you aware your Substar membership has been in "Payment Failed" for a while now? If they don't accept your current card, or you just aren't interested in supporting right now that's ENTIRELY fine, but given everything that's going on right now, and that you're still active and interested enough to post, I figured I should ask.