Just an alternative way of looking at it...ignore if you don't find useful!
Personally, I don't think buying things is too much of a problem (money permitting!). I think that the problem is keeping things when you have no further use for them. Sometimes it can be quite economical and ethical to buy something new, keep it in good condition and sell it on eBay when finished.
If you buy something that you use a couple of times and don't get on with it, put it on eBay/charity shop so someone can put it to good use. It is not a crime to buy something with good intentions and then find that it is not what you need - just get rid of it ethically (ie don't landfill it). Likewise, things you have used a lot in the past, if you no longer need, get rid to someone who does need.
I had a boy and then a girl. I felt a bit guilty about not reusing some of the boyish stuff for my DD so I gave all the boyish stuff to someone who recently had a surprise baby boy and she has put it to great use. I am really happy about this as those clothes could have just sat gathering dust and cluttering up the place.
I know someone else who was really busy with 4DC. She got in such a pickle that her entire garage was piled up to the ceiling with stuff, much of it was unopened brand new stuff that they hadn't got round to using. Also, some wedding presents from 10 yrs ago! Her kettle broke and she knew she had a brand new one in the garage, but could not face the mountain of stuff to find it so she had to get a new one from the shops. Decluttering is a great solution! Also, one in one out - if I buy anything (at all other than food/consumables) then something else must go out to eBay/friend/charity shop.