I don't think there's anything wrong to buy or use a car seat from a friend (most manufacturers will have the instructions available online or to order). And assuming that you trust the person in question that the carseat is not damaged or has not been in an accident. The problem I have is that people do sell old car seats on ebay, that may or may not be damaged, and may or may have been recalled, that may or may not have been in an accident and not be safe, and buyers have no way to know if the carseat is safe.
As for the nappygate, the rule on ebay is that you cannot sell used nappies, it's the rule. Some people do sell them and do get away with it because it's not possible to monitor every single item on sale. However, I have tried to sell used cotton nappies there and I did receive an email asking me to take off the item, and if I tried to sell used nappies again my account would be closed. It's the same rule for everyone, but I suppose just like every rule, some people don't know about it, or get away with it. I have quoted the rule in a message above, it's direct from the website. You cannot sell used g-strings, you cannot sell used knickers, you cannot sell used cotton nappies...
Oh and yes you can pass on carseats on Freegle, but they do have a warning message added each time a carseat comes up: MODERATOR MESSAGE: Please be aware of the dangers of using second hand child car seats. It is your responsibility to check they are safe to use. For more information visit here www.childcarseats.org.uk/ Thank you.
As for independent advise, I don't think it exists. But most independent advisors, including Which, the NHS, various charities inlcuding the NCT (which directs you to this website: www.childcarseats.org.uk/ ) "If you must use a second-hand seat, only accept one from a family member or friend (don't buy one from a second-hand shop or through the classified ads) and then only if you are absolutely certain that you know its history, it comes with the original instructions and it is not too old."
Maybe iabu, but I wouldn't buy a second hand car seat from a complete stranger.