I ended up on Mumsnet while looking for experiences from anyone using companies/agents that sell on eBay on your behalf.
I thought I would share my cautionary tale as I would have appreciated knowing more before trying it.
At the start of the year I decided to try out one of such companies which seemed to have very good reviews. I thought this could be a very convenient way to declutter if it goes smoothly.
The company in question only accept items that can sell for at least £25. I sent them a brand new with tags item with the price on the tag well over £100. The form for submitting the item had a field for the desired price I want it listed at. At first they listed it for a much higher price so I was happy with that but as months went on and no one was buying it, they kept reducing the price until it went below my specified price and then well below the minimum £25 value they themselves talk about.
At that point I contacted them to ask about the price I specified and to get the item back as their rules say if the item doesn't sell, they will return it to me for free (otherwise you need to pay £25 if you simply change your mind and want your item back). They said that I can only buy it back for £25 despite the item being repeatedly relisted for months. The catch was that their rules don't say how long the item should be failing to sell to be returned to the owner for free. So in reality you cannot get it back for free. When it came to the desired listing price I specified, they simply said I never set it but I have an actual picture and a printout of the form confirmation they generated with the desired price clearly specified there.
I ended up buying my own item from them on eBay as it was a cheaper way of getting it back and at that point I was curious to see what other rules were left flexible enough to calculate things in their favour.
Guess what % of the final sale price they took in the form of various fees 
.....80%! It was definitely higher than what I expected after studying their transaction and commision fees rules.
After paying some money, I got my item back and resold it for a very reasonable price as it's a good quality item from a popular brand so no big loss here (though I don't get how they failed to sell it for many months yet I had no problem). But it annoys me to think that if somebody else had bought it from them, I would have no item and would receive only several pounds at the end.
So lesson learned