Private party or retail?
I bring cash. $15k is out of my league but a couple thousand talks for me. For $15k I'd bring a friend to hold onto the money while I crawl around under the car. Your competition will have some excuse as to how they'll get the money Monday when the bank opens. You want to close the deal today, and so does the seller.
Retail, if you put money down and start a contract they'll hold the car. But they're just as likely to lie about it being there if you call.
If you spot something you like on FBM or CL, email immediately, and ask to see it ASAP, when does it work for you (seller), you (buyer) are available anytime tonight or tomorrow.
Don't try to talk someone down on price over email, do look sane, mention your town and phone number in case the seller prefers to contact you that way. If you want to haggle, look the car over. If the seller points out some faults, say those don't bother you, but "X" does, or just say it's not quite what you're looking for but could still offer "Y", right now, in cash. It is irritating and offensive to show up, where someone mentions a car needs tires in an ad, then say "yeah well it needs tires so all I can give you is "Z."
If a deal is good, the seller will be overwhelmed with responses. You want to be first, but not pushy.
If you message and don't get much feedback, the seller has another shopper coming to look, someone who was in line in front of you. They're holding onto your message as a "safety."