I've sold two old cars this year privately online so I feel like I have a bit of a handle on general stuff (plus I work in the marketing dept. of an auto sales chain). I would delete all of this because it's stuff you firm up after contact is made (and no one has read this far down anyway):
"important. . .no text asking if is still available will be answered
When it is sold the ad will be taken off
Ask any questions, tell where you are coming from, when you can look at it
Text preferred, calls ok and leave a message (include your name, where you are from, any questions, when you can look at it)
Will not hold, first $4000 cash buys it. I will text you when it is sold.
Cash only. No PayPal. No payments. No checks."
The whole tone of the ad isn't friendly or inviting, it looks like you're scolding any potential buyers and telling them what they need to do to make YOU happy. If someone is going to send you a scam text, a disclaimer in your ad telling them not to won't dissuade them.
Also, in my experience, the listings with "FIRM" listed on the price are the ones that sell the slowest, even if the price is fair. People these days (except in rare cases) will NOT pay asking price for ANYTHING. Start higher and then work down to an acceptable price.