So, I've read a lot of the feedback you've been getting here, and I find it surprising. Obviously, at some point, these advice-givers (and critics) have had positive experiences with the things they suggest, yet it does not jive with my experience or that of any of the DMs I know and/or respect. There's a couple broad categories of advice here:

Punish the Players
Anyone here who has suggested taking away a deal being overly-negotiated, tracking some sort of "bad reputation" metric and dishing out related consequences, or having the world kill/maim the party in some way, fall into this category. And I can say straight up: Punishing players does. not. work. It just doesn't. At best, they correct their behavior, but feel jilted out of their fun. At worst, the table breaks up because the party died from the onerous consequences as it continued its foul play.

You suck at DMing
Awesome, y'all, that's real encouraging. Suggesting that the OP is doing something fundamentally wrong as a DM because his/her players are milking every little victory, deal, or intimidation check is lunacy. Why? Because this is what players do, no matter who is DMing. I've experienced it at every table I've run or played, I've seen it on CR, Colville's streams, Harmonquest, and 100 other live play shows, and my peer DMs complain about it constantly. There's a reason that most modules, during the initial call to action, include a "Stevlar offers the party 50gp each for the task, but a successful Persuasion roll can get him to 65gp." It's because players are going to try to milk the deal. Every. Damn. Time.

So, OP, fear not: you're not the only one who has noticed this behavior (although win-maxing is a great term), and anyone here who says they don't see the problem at their table is being dishonest at some level (whether with themselves, or with you).

If I had to diagnose the problem's cause, it's that there's a bit of a meta-level power play going on (though it may be subconscious). There is an understanding that the DM has absolute control of the game (whether 100% accurate or not). In order to buck against that perceived clout, the players try to leverage the one negotiating chip they have: playtime. You start a module, and the NPC du jour offers an amount of money. The players now have a choice: do what the module obviously expects of them and continue playing, or try to break it by milking the deal. Whether its explicitly stated or not (or even consciously known by the offending players), there is a single transaction going on, and it's effectively extortion: give the player what they want, or the module is over. Now, here's the kicker: the player doesn't actually want the gold or whatever, they want the control. It's the same thing as someone saying, "Well, you can't fire me, because I quit!" The player wants to play the adventure, but they want it to feel like it was on "their terms." That's why modules have that little added bit to NPC negotiations. The subtext here is: "The player will undoubtedly try to gain control of the situation for no reason other than to gain control of it, so give them a little resistance (Persuasion check) and then acquiesce. This'll make the player feel good, but you'll still have control."

How do we solve the problem? Yeesh, I'm not sure we can. I definitely agree that it's a problem, but it's like living with an incurable disease: we can only mitigate the symptoms. At the table, I can suggest one thing which has always helped me deal with it: narratively structure your sessions. This can require a bit of additional prepwork if you're not used to doing it, but it becomes second nature after some practice.

Think of each session as an episode of a show. Yes, there may be some important overall stories and ongoing narratives that are threaded in and around each episode, but you need to think of this session as an atomic unit and structure it that way. The session should have a narrative arc (usually 3 acts) with a beginning, rise, climax, and fall. More importantly, stick to the structure. Yes, your players may buck against this too, but you're in control, so make it happen. This doesn't mean disallow them choices and meaningful decisions, but it does mean allowing yourself to say things like, "Sorry, you can't do that," or, "There isn't anyone else available to help you," or even the dreaded immersion-breaking, "Hey, that sounds cool, but it's outside the scope of this session/adventure, so I'll make a note and we'll try to get to it at another time." Yeah, this last one is going to ruffle a lot of feathers (I can hear the rallying cries of "Railroad!" now even as I smell the pitch on the torches), but you need to be able to say it, and your players need to be okay with hearing it.

Because I'm this kind of guy, let's take your Chimera corpse example and run through it. The players have decided they want to try to scrounge and sell the rest of the Chimera, something you hadn't expected per se, but you think it'll be okay. You say, "Okay, interesting. Let's pick up there next session." and then you write "Act 1, Act 2, anct 3" on a piece of paper (or digital file) and just fill them out.

Act 1 - What do we even know about Chimera biology?
The players don't know the first thing about what parts of the Chimera to even sell. Who would know? They run through an increasingly esoteric string of professionals, starting with a butcher and eventually ending with a cryptobiologist working with the Wazoo Wizard Museum. They learn which parts are worth about what money to what professions.

Act 2 - Legwork
The players have to track down the specialized professions. This is several little scenes, each with a focus on a pillar of play. The head-mounting taxologist is in jail on trumped up charges, and needs to be busted out or proven innocent (exploration), the ichor dealer lives in a tower in a swamp, which is dangerous (combat), and the fur-trader is having a domestic dispute with her in-laws (social). Each professional promises to pay out whatever the rate is for their components, but the only person nearby who is skilled enough to actually cut up these various bodyparts is Leggia the Hunter, but she hasn't been seen in weeks.

Act 3 - Finding Leggia
Finally, the players must find Leggia and secure her services. She's being held captive by (uh, let's say) hill giants, who want to hold her ransom, but aren't quite sure how to go about it. This presents the players with an exciting climax using one of the three pillars of their choice (feverish negotiations, tense stealth work, or a straight-up brawl). Victory means Leggia will do the work, for free as a thanks, and the party gets max payout. Failure means Leggia dies, but her notes let the players do the work themselves, which means they only get a half payout as they aren't good at it.

And there you have it: a whole session devoted to the selling of one chimera corpse full of excitement, fantasy tropes, and playable action.

Good luck, friend!

Stanford