How do I deal with players always (greedily) pushing for higher
You deal with that by giving them what they ask for.
First ask yourself if it truly breaks the game. If the game would be ruined, then just say no. Otherwise, and if it would make the game more fun for the players, then you could just say ok, since some players are happy to be ahead of the average gold for their level or the average power for their level.
If they are asking out-of-character, such as "Aw, that sword is only +2? That's barely an improvement, and the current sword has been used for 2 in-game years so my guy is attached to it. Can it be +3 instead?" then that player probably wants the game to be a bit higher powered.
I've been there, and I once offended a DM when he gave me a +1 sword minutes into the game at level 1, when I asked if it could be +3 or at least +2 instead, he said what he gave was more than generous, and the fight was in a town so I said "Ok, then I sell it immediately and buy a nice suit of armor instead" (the armor would have been just a normal, non-magic one, but a decent one). The DM was upset and insisted a +1 sword gift this early should be respected, and he did not care when I pointed out that all it would get me was a +1 on my attack and damage rolls, while the armor would increase my AC enough that it was way more useful.
For the in-character stuff, that is not even a problem. Amassing a fortune of gold is not a problem. This is something I usually do when I play. I often skip buying equipment that other people might consider "stepping stones". Buying a +3 sword when you already have a +2 sword really doesn't give you a large boost, and the average attack is only marginally better. Not worth it in my opinion. I might even skip more than that and just hoard my gold until something I think is worth my money comes along. After all, it is my money (well, my character's, who is an extension of me, not you).
The only problem I see with what you have described was merely in how rude the character was to the quest giver. But hey, depending on the disposition of the characters, maybe it was appropriate. Or, if you don't want to be strict about reacting to the exact way in which the player said it, then just overlook the rudeness and act as though the player asked it a nicer way.
For example, some players don't like roleplaying first person because of their social awkwardness and prefer roleplaying in the third person instead. Instead of "She's worth more than that! Give us 2000 gold and we'll call it a deal." they may say instead "My guy thinks that she is worth more than that and negotiates for 2000 gold." The first option sounds a bit rude, but the second option can't really be seen as rude because you don't really know what exactly the PC said or how they said it, only the type of interaction that was made.
If you treat the former as rude and punish the players for it, you might get a lot more of the latter, people not wanting to put the emotion and first person role-play into it. So you could just treat all instances of first person statements as if they are a generic command and not react to any possible emotional connotations involved, unless of course the emotional part is specifically intentional on the PCs part.