The guy asking for your phone number could be a flake that doesn't want to give you his phone number out of fear that you're some scammer, but it could just as easily be someone fishing for phone numbers. Either way, you don't want to deal with them.
My advice is: Do NOT reply to anything that you feel is hinky or makes you uncomfortable. You do not owe them a reply; do not reply out of some misguided sense of "courtesy."
It is okay to screen your calls. Numbers that you don't recognize, let go to voicemail. If they are a scammer they will not leave a message. If they are serious they will leave a message. This will also give you a second line of defense against flakes, as you can usually tell by the tone of the message if they're going to flake out on you.
Also, don't put extra stuff in your ad like hoops for the buyer to jump through, code words or rants about scammers. That just makes the legitimate people NOT want to do business with you. Scammers are not going to stop trying to scam you just because you told them not to in your ad.
I screen all my responses. All, "Is it still available?" responses go in the trash. 99.99% of them are either flakes or scammers. I will respond to someone who is polite, takes the time to ask something specific about what I'm selling, and at least makes a little effort at spelling and grammar.