How to say no?

So I want to start off by saying I grew up in a family that didn’t have much. I mean we got by but we were definitely not average. I always worked from the age of 13 and tried to make myself presentable and hold myself to a higher standard. I love my family and am not ashamed of where I come from but I want more than that. I don’t get my hair done or nails done but I spend extra on quality items. Now that my fiancé and I are pregnant it seems like I’m getting judged for this way of life. I live in Australia now so it’s just his family around and they keep offering us second hand bad quality items even though I nicely said prior no one is to feel obligated in buying us stuff and if they want to we have a list. Every time I say no they get offended and act like I’m a snob. My in law even told me I should make a baby seat out of a pool noodle instead of buying one. I’m really trying to be minimal but quality. We plan on having multiple children so I just want to buy quality items and reuse them. I don’t know how to keep saying no to his family. They usually ask me behind his back so that I feel obligated. I’m not prissy but I know I’m coming across that way. His cousins are always dirty and uncared for and that is not how our child will be. They always make comments about us being uptight but everyone insists they are joking. I don’t feel the need to bring myself down to that level. It isn’t funny to spend your money on your drinking problem and buy cheap crap for your children. I feel so out of place but I’m standing my ground on what I want. Can anyone tell me a quick way to get them to stop asking and not cause a feud?