I’m not sure whether I can answer all your questions, since it depends on a lot on what you buy, where you buy it from, whether it’s an official middleman service or just a person helping you out, etc.
In general, a middleman is primarily responsible for receiving the product from the seller. Checking if everything is there, and if not handle some problems with the seller. And then repackage everything to be shipping out to the buyer.
I personally use or have used multiple middlemans from various parts of the world. I personally live in The Netherlands (Europe), and collect Pikachus in all 14 languages.
I’ve used an official middleman service twice: one for Japanese purchases from Yahoo Japan (for which I’ve used the middleman service Noppin at least fifteen times without any problems thus far - more info about that here); and once with the Korean middleman service Harumio for some Korean Pokémon webshops like tcgbox.co.kr. Both of these companies are professional middleman services whom I can recommend.
But note that they are a general middleman service, and know nothing about Pokémon in particular. If you accidentally bought a fake Pokémon card, or a 1st edition instead of unlimited, but it’s still the same looking card, then the middleman service wouldn’t know any better than that the seller send what you have bought. I haven’t experienced this myself, but it is of course a possibility. It’s also harder to ask Pokémon-related questions to these kind of middleman services, since they for example don’t even know what ‘holofoil’ (or ‘kira’ in Japanese if I remember correctly) means.
I’ve also had multiple online friends helping me out from various parts of the world. One who helps me almost on a monthly basis for purchases from the US from sellers that don’t ship worldwide; but I also have a middleman in Brazil for Portuguese cards; in Italy for Italian cards; in Indonesia for Indonesian cards; in China for Chinese cards; in Germany for German cards (in the rare case a German seller doesn’t want to send to The Netherlands for whatever reason - since 99% of the German sellers I contact will also ship to the neighboring The Netherlands without a problem); and one in Australia.
If you can find someone trustworthy online to help you out instead of an official middleman service, I would recommend to do so, since it’s a lot cheaper, and other collectors are also more knowledgeable about the purchases you make through them. They can also make pictures of your purchases, so you can more easily verify if everything is in order.
Here are some Japanese middleman websites listed, including Noppin. But usually it’s indeed recommendation by hearing it from others. There are also loads of members here on the forum whom help out when asked. I’ve also helped out some members myself from the US who wanted to buy foreign cards here in Europe.
Always the buyer. In most cases, the seller won’t even know that there is a middleman. Let’s say I want a certain Portuguese card, then I ask my Brazilian friend to search for it. So he’ll be the one buying and receiving the card from a seller, and will also leave feedback to that seller. And I will (usually before he makes the purchase) send money to him for the product itself + shipping from the seller to him + shipping from him to me - although I usually combine multiple orders before he sends them out to me.
It’s also possible I as buyer am in contact with the seller, and I ask the seller to send to my middleman’s address. In that case I’m the one paying the seller, and my middleman is only responsible for checking if everything is in order, and forwarding it to me (after I paid the shipping fees from the middleman to me). After I receive it, I will then thank the seller, and leave feedback. This second case is the exception though, since I usually just ask my online friend that I’m looking for a certain card, after which he will search for it at his local webshops.
Official middleman services will state their percentages at their websites. Noppin for example is more expensive the higher the purchase price, since it’s percentage based. I once won an auction for around 1500 USD for example, but still had to pay ~150 USD middleman fees on top of that. And then I haven’t even mentioned the shipping fees and import fees on arrival.
Some people say buying Japanese cards from the Japanese market is cheaper than let’s say eBay, and in general this is indeed correct. Some cards that are offered for 100 USD on eBay, can be found for 70 USD on some Japanese secondhand webshops. But in my experience, it’s usually still cheaper and easier to buy the one from eBay due to all the extra fees (middleman fees, 2x shipping - from seller to middleman and from middleman to me, import fees, etc.) that has to be added to the initial product price.
Usually my online friends help me out for free (by which I mean I don’t have to pay anything more than they’ve paid themselves), and I usually help them out for free in return as well. So I give money for the product itself; shipping from the seller to them; PayPal fees; and shipping from them to me.
Hope this helps a bit. It’s not too easy to answer your question, since there are a lot of different scenarios and variables to consider.
What would you’d want to use a middleman service for, if you don’t mind me asking? If you have a particular reason to ask these questions, it might be easier to explain things based on that.
Let me know if you have any other questions, and I (or others here) will try my best to answer them. 
Greetz,
Quuador