I've ordered several times through their proxy service and even had them bid on some items before for me. So far, everything was always handled professionally and packed really well.
Their shipping is a bit more on the expensive side compared to some other companies from Japan I used but their fees are definitely incredibly cheap and their service is great.
I'm collecting figures and merchandise of various games, anime and manga and some things is limited to Japanese stores, so unless I want to pay horrendous amounts elsewhere, a proxy is more or less the only choice.
Zenmarket is great for that since they are fast, reliable and always try to solve problems in your favor if possible.
There was once an issue where I had pre-ordered a figure but when it was released, they didn't get it. It was a limited figure which had to be pre-ordered within a limited time frame so getting it elsewhere for a decent price after release was virtually impossible. But they talked to the retailer for me again and again and after 3 months, they managed to receive it from the retailer after all.
A normal seller might have just canceled the order due to not receiving the promised stock, refunded me and I would have been left without anything to do about it, but they didn't give up so I was able to receive it, even if a little later than planned. But that was hardly their fault since they always procurred items reliably for me before.
Their fees, as mentioned, are pretty cheap. They only charge 300 Yen (so around 2€) per item and then charge transaction fees for the payment method of your choice. Which is reasonable because these services aren't free for them either.
Other proxies tend to have lots of fees to them but Zenmarket is very upfront about what you need to pay and there's nothing else hidden.
They offer various other options I haven't used so far like additional reinforcement for parcels, but their packaging has always been extremely secure anyway so I never needed it. They treat the goods really respectfully and make sure everything is securely packed regardless of additional options to pay for. My figures for example always came in a sturdy cardboard box, sometimes repacked for free if they deemed the original parcel unbefitting or too large, wrapped in bubble wrap and secured with a good amount of newspaper, etc. to make sure even the box wouldn't receive any damage. For collectors, this is indeed pretty important. Of course they can't be held responsible for what happens before but they try not to provoke any preventable damage.
So all in all, I can safely recommend them. The staff is also fluent in English (and likely other languages), so they won't have issues understanding you.
They also work with a balance on their website, likely to avoid PayPal disputes directed their way. It's a common practice for proxies since they don't want to deal with problems that are not their fault and lose money in the process. They will still help you solve problems of course, but they just don't want you to claim their money for issues the actual seller caused. I don't take this positively or negatively. While I would like to pay immediately, I do understand their perspective since they are really only the proxy and don't want to be held responsible for defective, etc. goods that were already not matching the description when the item arrived. They won't open item boxes to check figures for example.
-Fast, professional and reliable service and customer support if needed.
-Fees are cheap and there are no hidden fees.
-The website is pretty simple to use in my opinion.
-Allows for bidding on certain websites with sniping bid options.
-Secure packaging and reliable shipping.
-Generally very informative. If you pre-order an item, they will ask you if it's okay and whether you've seen that it doesn't release until later.
-They have an auto-translating software for items accessible from their website. It has to be taken with a grain of salt of course but it's helpful to get a general idea.
-Shipping is a bit more expensive than on other websites like AmiAmi, etc.
-The shipping calculator isn't always super accurate.
-This point isn't anything they can help, but some websites seem to be blocked for them because these websites already have a partnership with another proxy service. For example, Mercari.jp always gives me an error message that they can't buy from this website because they made the experience the seller doesn't want to deal with proxies. I do believe that but I also think these sites just refuse to do business with other proxies dud to partnerships with others. Not Zenmarket's fault but if you see it, thst might be the reason.