It's so funny how people have different definitions of rude, you can't win!
I think it probably comes down to how you see no-longer-needed items.
If you see them as valuable, then of course it's a gift to pass them on to somebody and I can see how people may feel the value should be retained either by passing forward as a gift or offering back.
If you see them as clutter/junk, then it's a favour to the giver for the recipient to take them away, and you certainly wouldn't want them back again afterwards! In fact I've seen threads where people are annoyed at having things "dumped on them". TBH, I definitely would have assumed this about a cot and changing table - these are bulky, annoying items to store and fashions/safety standards change. I wouldn't want to use a 30 year old cot and changing table for my children so it wouldn't occur to me that somebody would want to put it away for their children.
I think it's universally rude to ask for something for free and then immediately sell it without even using it, unless you disclose that you are doing that (e.g. charity shop model).
But if somebody has given you something without saying whether it's valuable/clutter to them, then I don't think it's necessarily intended as a grabby thing, if the recipient has thought oh, giving-person doesn't need/want these any more, it's doing them a favour to get rid of them.
Also I have found it can end up being a hassle if somebody tells me "You can have this but it has to be passed on within (certain group)" - what happened then was that COVID happened so no new people joined the group and nobody had any babies. I was stuck with this stuff for ages until eventually I just donated it. Likewise my aunt passed Brio train tracks down to my dad for his second set of kids and he then passed them to me for DS1, but with the idea that they will be passed back within the family, so one of my siblings or cousins in case he has children. But we have since moved country, had 2 more children, the train set has been added to so much it's unrecognisable. It's nice that it's been through so many children but honestly we could have just bought new and I'm wondering if it's going to end up being a huge hassle to try and ship it back to the UK when DS2 and 3 grow out of it!