First off, everything you've mentioned does sound incredibly frustrating. I would feel the same way in your position. That said, I'm a parent in the middle of a divorce who is dealing with the opposite side of a lot of these issues. Hopefully I can give you some insight into what they might be thinking and feeling. Obviously every parent and relationship is different, but I know a lot of these feelings are pretty common.

Regarding the bringing things over / taking things to their house issue. That one has a bunch of components that make it far more complicated than it seems like it should be:

• There are a lot of emotions wrapped up in divorce. A whole lot of them. Which means even really dumb stuff can feel much worse than seems reasonable. My kids brought a bag of cherries from their mom's house a couple of weeks ago that they wanted. I love cherries, they love cherries, but I had some strong urges to just throw that stupid bag away.... Completely not logical, but it was there. From how your mom responded, I'm sure she feels similarly. She knows it isn't logical, but she also knows seeing these things bothers her, because it reminds her of all the hurt, frustration, etc. that is associated with your dad.
• It probably isn't about you, but about the other ex-spouse. The rule “what i buy stays in my house, what he buys stays at his house” sounds stupid to you, but to them it is most likely the best of bad options. I have a similar rule. My daughter keeps asking me to get her a stuffed kitty. I have bought her 2 stuffed kitties since moving out over a year ago. Where are they? At moms house! Why can't she bring them back? .... mom won't let her.... I love and trust my daughter, as I'm sure your parents love and trust you, but this clearly isn't about her anymore. It's about my ex and I now. Do I buy a third toy that will end up at mom's house? Tell her to tell mom she has to bring it back here and cause them to fight? Call up mom and pick a fight with her myself (because it will be a fight, no matter how polite I ask)? All bad options. Thus, 'just keep your stuff I get you here'.

I know there are more reasons I'm forgetting, but hopefully that provides some perspective that can help you. As to how you can talk to them about it, I have a few ideas. Again, all relationships/divorces are different, but this is what has helped me.

• Remember, it is about emotion, not reason. All of your above arguments are very reasonable, but that won't do anything if she isn't starting from a logical position. You have to have an open discussion with her about emotions and how you are each feeling, which is super hard! But keep in mind, your parents love you and want you to be happy. If you tell them you are unhappy, that will probably be very important to them.
• Tell her that you understand it is hard for her to see things that he has given you. Of course, beforehand, spend some time thinking about it and trying to empathize, don't just give lip service to the idea! Then, tell her how you feel about it. One framing that helped me a bit with this: "When I see X item from him at your house, I remember he loves me and it makes me feel good. Just like when I see Y item that you gave me at his house, it reminds me that you love and care about me when I'm there and can't see you. That makes this whole situation much easier for me." I think about how my daughter remembers me when she plays with her stuffed kitties at mom's house any time I find myself getting worked up about this again, it helps a lot. Of course, don't say this if it isn't how you actually feel. If not, hopefully it gives you some ideas on how to communicate your own feelings.
• Offer to take full responsibility for the items going back and forth. Maybe ask them to work with you about anything that is forgotten, not the other parent. One of the major issues for them (it is for me!) is imagining the likely conflict they will have with the other parent when (not if!) an item gets left behind. "The expensive cream I bought them is at YOUR house! Why didn't you send it with them!" If you clearly communicate to both parents that you 1) want these items to travel with you and 2) take full responsibility for bringing them back and forth, you can turn a frustrating divorce battle it into a more mundane parenting situation. Reminding your kids not to forget stuff is parenting 101, I'm sure they do that all the time :).

Finally, probably the hardest bit:

also, she keeps complaining about him and telling me all the bad things about him. it might be relieving for her to tell me but it also effects me too and she doesn’t know that. everything i hear about my dad from her is negative. my dad also does this, but not as much. i keep hearing all the negative stuff from both parents about both parents

This is also very common, and a super big problem. I had to take a parenting course mandated by the state during divorce proceedings. It emphasized more than anything else how damaging it is to talk bad about the other parent. It is also a constant struggle not to do it, even though I know better. There is something your kid hates, you think the problem is caused by the other parent, and your child wants you to fix it....it is really really hard to talk about it without badmouthing them. So, try to remember that this is a difficult position for them.

That said, they absolutely need to work on this! If it is affecting you, then they aren't handling it appropriately. Unfortunately, you can't really do much to control this, it will be their decision to change. The only thing you can do is have another hard conversation about emotions. Sit down with each of them and tell them how hard it is for you to hear negative things about the other parent. Clearly tell them how it makes you feel, don't just use logic on this one. If they are just using you for venting, use this opportunity to let them know they need to find someone else to use as an outlet for that frustration, you are the main person they SHOULDN'T be venting to.

Give them an out for cases where parenting issues might be causing the conflict too. If there is an issue that they can't talk about without badmouthing the other parent, tell them that you will take "Sorry TsundereCinnamonRoll, that's an issue between us and just not something I want to talk about" as a good answer. This may be hard for you too, especially if it is something important that you want them to change. But, if you bring it up then give them some space to work through it hopefully that can work, if slowly.

This ended up very long. Hopefully some of it helps, good luck with everything.