1. All prices are negotiable.
Duh. The number in the listing is a starting-off point, clearly. This is a very subjective process and all of these prices are very elastic. If it's winter, bikes should be cheaper, because the demand is less. Know this. You could check on eBay to see if the seller listed the price reasonably, but remember that it's not that simple because eBay gives you guarantees. Craigslist is anonymous and offers no protection. That risk means you should pay less for a comparable item. Some people like to talk about a 15 to 20 percent rule when negotiating, but that's mostly psychological. Prices are too fickle to pin it down to a number. Do your research.
2. You don’t need to buy. You’re interested, intrigued, looking to buy, but not in a hurry. Browsing, but serious about it. Sadly, dating and negotiating have things in common. Desperation isn't a good look and will keep prices high.
3. If the posting doesn’t have all the details you’re looking for, send a friendly email. Being polite is important. If the seller has an incomplete, unhelpful listing, it narrows people down. Less interest means better prices. Sending an email costs you nothing and opens a channel which fosters trust. The seller has invested in you if they’re responding.