@aSofaNearYou - I have this situation with my ex. We do a “swap back” about once a month, and I know I don’t get a lot back (whereas his are easy for me to find as I bag them straight up and they are always from Primark so I can check labels) even sometimes brand new things I’ve bought that I’ve only seen them wear once.
But, I accept it by telling myself that they will get the use of those nice clothes while they are there ( although I do have to chase coats, P.E coats and shoes as can’t keep replacing).
My ex will literally send them back in pyjamas and bare feet, so that I don’t get any of “his” clothes, and if they ever do have them, my DC panic when they take them off and start saying “Dad wants these back” straight away.
I don’t want them to feel like that, so I say nothing to them. I just remind ex if it is something I particularly want them to have (co-ordinating sets mainly, when I only have half a set!) Sometimes I get them.
I was advised, by a DV counsellor in the past, that his behaviour around keeping needed things is about control (as I have to keep making contact to ask for them). She advised me to get a bag of charity shop clothes and send them in those, which I did back then but don’t bother with anymore.
As they are getting older and are more into fashion, they will sometimes hunt for an outfit here that they can’t find. I just remind them that if they want certain clothes here (where they are most of the time) then they need to pack up and bring them back themselves.
I hand over wetsuits, bikes, phones/iPads - and sometimes feel annoyed that I have to track down to get back. But it’s about what is best for DC, not about disputes between me and the ex.
As it happens, he tells everyone that I steal the clothes he buys, won’t take them to activities (one that he booked at 5pm, without asking me, when I work full time in another town, then told DC that they couldn’t go as I refused to take them! Again, about control), and that I feed them take aways all the time and am generally neglectful. None of this is true.
If it wasn’t for different circumstances then I’d have wondered if I was ex described in OP. 
It’s a common script though, from abusive men who lost control of their former partners.
I’m not saying this is case here, but I do tend to take stories of neglectful ex-wives with a pinch of salt. Particularly when the father hasn’t even fought for 50% custody, despite telling everyone what a terrible mother the ex is.