What would you do in this situation (selling a car) what do I do?

I have a 99 ford ranger extended cab 4 door that is up for sale on FB for $3500. I had it checked last month for any discrepancies and they fixed everything, rotated ties, etc. Car has never been in a accident and has been maintained on time. The only current issue is a 6th cylinder misfire issue. The car is spotless… No rust even. I had multiple people interested in the car, but were wary of the shaking due to the misfire so they never got back to me. This older lady contacted me yesterday asking me if I would let her mechanic check it out before committing. That sounded like a smart idea (if I were a buyer). So I brought it, the mechanic drove it around, and came back and said it 'misses', otherwise in top shape. The woman askes" well how much to fix?" he said "well, depends what is wrong. Maybe electrical, maybe the valve. But I don't think it is a valve" and the lady kept asking how much it would cost just to get a diagnosis… NOT to get it fixed and he was being vague and said "could be $200, could be more or less" so she started doing math in her head and took "$200" to heart and said to me "will you go down to 3000 and I pay the bill for the diagnosis?" I thought that was a decent deal I guess and she was interested.

Added (1). I gave my keys to the guy this morning to get it looked at and now, today, I got a random message from the lady asking why I wasn't at the mechanics. I said "I dropped off the car this morning, I'm at work till 8 and haven't gotten a call yet" she goes "well they finished fixing it"… Why did she get a call before me? And why did they just go ahead and fix it?

Added (2). Apparently she called over and over again till they budged and told me "well I figured since the car was already there, may as well fix it too" so SHE gave THEM the go ahead saying she was going to be buying it soon anyway.

Added (3). I called the guys and they were already sick of talking about the car and I hadn't even called them before! They told me it was not "done" yet, but they found it needed a 6th cylinder fuel injector piece and that was the issue, he wouldn't tell me the price it would be but said it would be ready tomorrow morning and he would call then.said I would be around $360.

Added (4). I messaged the lady after and said it would be $360 so she did more math and asked if she could have it for $2500 if she pays the $360 fee. I said no and she was like "please. For $2800? I can't go higher since it was more than a $200 fix" and started ranting about a loan she is desperately having trouble getting already cause someone stole her purse last month and blah blah.

Added (5). I don't even know what to do… For 1. I didn't want to go under 3k FOR THE CONDITION IT 'WAS' IN. Now that it was fixed and in tip-top for 300-400 I feel like I'm in a quandry, and jipped quandry cause she is using the price of the fix as a bargaining chip. And I don't have the money to pay for the fix even if I wanted to try and sell if for the going rate for the truck fixed up… Which is around 5k.

What do I do?

I would have pulled the plug on #6 and checked it, the wire and the cap.
No I wouldn't pay for the diagnosis since it was half a $$. You knew it missed.
If she is paying for diagnosis and you want the truck gone. Do it

Pay for the fix, and repost it for sale - at your original price- no one will be scared off by a GOOD running car - it was a mistake to try to sell it with a miss anyway - so now that it is fixed top dollar should be expected. Her loan issues are not your problem - move along - she sounds like a basket case anyway.

You had the tires rotated, but did not bother to fix the miss. (?!)
You agreed to take $3000 for the truck as is. (she pays for the diagnosis)
Now she should pay for the diagnosis (as agreed) and $3000 for the truck (as agreed)

The problem is that (i bet) that the agreement is not in writing. The shop fixed your truck but without your authorization (by the sounds of things, but I do not know what you signed when you dropped it off). You can argue about this, but they are holding your truck.

You do not know if she will ever get the loan, but if you cave and agree to $2800, then what do you do if she can't raise it.

Best to pay for the repair and tell the lady that the price is back up to $3600. Good chance she will cry and wring her hands and pull her hair. Then you raise the price to $3800. Eventually sell it to somebody else for $4000.

Any deal should be in writing.