This is a good question and it would be largely dependent on your location. Such as if you are in the UK or Canada, where shipping to the US is insanely expensive.
Ebay really is the only game in town for selling collectors items online. Amazon works but they charge huge fees and then just let people steal stuff when they say it wasn't received. Amazon is pretty much a massive con job all in their favor.
The #1 best place to sell collectibles would be at a booth at a local convention. My husband and I have been doing it since the, well, don't want to reveal my age. A long, long time. In fact we started selling direct on Outpost Gallifrey decades ago.
We try to do at least two conventions a year and well over 90% of sales are driven by the excitement of being able to see, hold and buy what you want on site. Covid really hurt us because of the events cancelled.
Outpost Gallifrey does not appear to have a marketplace like it used to years ago. From what I am reading there do not appear to be any rules so to speak of, so why not just create your own marketplace here? It may be better to contact an admin first but I don't see any reason they woould day no. Just post what you have for sale here. I'd like to see it. And if you take Paypal you're covered with a tracking number.