Oh I've got a big list of these...
Baby baths. Perhaps we didn't buy the right ones. Either too small, too big, too slippy, difficult to hold DS etc. We liked the shnuggle for a while but DS grew out of it so quickly. In the end I gave up buying baths and just put DS 4 months in a shallow bath and he loved splashing around. Plus we freed up space around the bathroom.
Baby towels. DS grew out of all his baby towels by the time he was 6 months old and then they just became annoying small towels floating around the linen cupboard. Since then DS gets wrapped in a regular bath sheet.
I bought a ridiculous blow up baby chair from mamas and papas. I bought it because I went to another mum's house and her baby had so many pods, rockers, bouncers, play mats, and this blow up chair that looked lovely. I hadn't bought my DS any of that stuff and I left feeling bad about myself (and for DS!). So I rushed off to buy the same chair. Which was useless. He couldn't sit up right in it, and if I took my eye off him he would slide all the way flat on his back before shouting for help. The toy board at the front bored him within 5 minutes. He then figured out how to rip it off and then because he had no balance would just face plant onto the carpet. By the time he could actually sit in it properly he had already began crawling and had no interest in being confined to the chair.
Chicco Polly magic high chair. This was actually a gift so I feel a bit guilty saying this. Firstly it's enormous. Folds up but it's still enormous. Prone to falling over when it's folded. Doesn't really prop babies upright enough for them to be safe while eating. The pleather upholstery plus all the nooks and crannies are a nightmare to clean. And it costs around £170! DS now has an IKEA high chair and it does the job.
Baby blankets now that safe sleeping advice is to use sleeping bags. I've got a drawer of blankets that aren't used.
Soothers. Was gifted and I bought a good selection. Likewise the safe sleep advice is to not let them sleep with any toys. So they also just sit in a drawer.
Steriliser (mam 5 in 1). Took up too much space on kitchen counter, needed cleaning and drying in-between uses. Just a total faff. Within a week I'd packed it back in its box and was using either cold water sterilisation or steaming stuff in a pan of water.
Nappy bin. Of all the jobs I didn't want to have to deal with, emptying the nappy bin topped them all. So we got rid, and nappies just go in the regular bin and I had one less hassle to deal with.
Changing mat covers. My DM bought us a selection of white (!) fluffy changing mat covers that basically had to be changed as often as babies nappy because virtually EVERY poop was a poonami up to his neck and would get the covers dirty. Or, if I was trying to protect the changing mat cover, I would be trying to yank the damn thing off with one hand while holding a poo drenched baby in the other. Again, just an additional hassle to deal with. In the end, if DS needed to be laid down naked on the mat, we just used a clean tea towel or muslin so he wouldn't be cold.
Breastfeeding pillow. The only person who used it was my husband while ds slept on him. I just found it annoying and it got in the way of easy position changing while feeding.
We could have saved a lot of money. Hindsight is 20/20 I suppose.