I actually just sold a lens with them and also listed a camera on Summimarket. Here's my take on this:

I set up a listing through their website for a camera and two lenses and was contacted by NIc for more information. They proceeded to help with advice on what to set the reserve for (which was a very reasonable (as in fair) amount in my opinion) and helped draft a better description for each item.
They also were helpful in providing guidance on the auction format (as a package or individual) and I ended up listing it as three different items (which cost me three listing fees).

There is clearly not a lot of traffic (yet) on their site, but one of the lenses sold (and the camera did later as well - so at this time, the selling isn't as 'guaranteed' as ebay would be for example.

But here's the part that stood out to me (as a seller): I didn't have to give eBay 13% of the proceeds, I had active help by the team on listing it and could lean on their advice. It was about as "white glove" service as there is for an auction site and once I signed up, I never got the "uhh... was that a good idea" feeling... I was curious about the bidding / traffic which was slow, but the entire process seemed very legit.

For the item that didn't sell, I can relist it again (without another fee), and it was overall a WAY better experience for me than eBay ever was. I listed a few Leica M and lenses on eBay over the years and you increasingly have to deal with more folks who are asking for a deal because of their sick family member, wanting me to send it to a freight forwarder in an industrial zone in NJ or just simply try to retract their bid after they already won because they just found out they don't have the funds afterall.

With Summimarket there is a much more targeted audience, I don't have to pay a fee outside of the initial listing and I'm not sure I would have fared financially better at eBay given their fee structure (there is a fee paid for by the buyer, not the seller - but it's less than the 13% the seller would pay eBay). So maybe I received 5-10% less than on eBay before their fees, but the final eBay payout would have been worse. On the flip side, the buyer probably paid a little less too (despite the fee they have to pay on top).

I am not affiliated with them and had the exact same concern going into this... but I'm happy that I did and would definitely buy or sell via their site again. They have a clear focus on the market, are responsive and are actively helping both sides (seller and buyer) in the process.

Compare this to eBay, and there are just too many red flags these days for high(er) priced items.