No worries, happy to help. I have MSc Computer Science and 25+ years web dev experience, there's not much I can't do (so long as it isn't Microsoft - don't touch that stuff), so if you would like to talk about this then please contact me privately.
For what it is worth, I trust the MailChimp group, have used them over the last, um, maybe two decades? They are consummate professionals and take security very seriously. In fact I would argue that by not using a service like this you are taking greater risks with user data and possibly missing the mark when it comes to complying with GDPR - maybe?!
There may be a problem in that your terms and conditions may not cover using this kind of third party service too - needs to be checked.
All sortable, but requires careful thought and consideration.
Also, never used them but I hear good things about MailJet and their pricing is pretty reasonable.
I would definitely move the marketplace to Unicorn Land - that gets my unreserved approval - if you can afford modules you can afford to support Modular Grid - end of story!
I would however make this so only Magical Unicorns can sell, but anyone can browse and buy, that's fair I think!
It also has the side effect that there is a much higher chance that a seller is a genuine person with a traceable email address, PayPal account and bank account. It helps with accountability, not that it is needed much around here, I have had great experiences with everyone so far.
I wish you all the very best and as I say, if you would like to chat more about this please do get in touch.
Kind regards :)
Modules for sale:
Intellijel μVCA II
Endorphines Grand Terminal
You can find my modular synth album here https://kel-audio.bandcamp.com