I would suggest Aquabid as well except for one big difference. The sanity level on Aquabid puts it totally out of the Craigslist realm! On Craigslist one has to be very , very careful. I've never met anybody trying to scam me on Aquabid.
This would be an issue determined by the indiviual rather than the site. Many are full time dealers but there are also many general hobby sorts who sell their excess stock. It is always a good idea to verify how things will be shipped before buying. I don't know of any meds that are used routinely for shipping but there are many times when different items are used to help maintain the ammonia levlels so the fish arrive in better condition. The sellers I have bought from were just advanced hobby guys and did a very good job. The only disappointment I have found was more a timing mistake. I ordered during the Christmas shipping rush and a two day priority mail took six days. Not something I could blame the seller for doing but in talking to him, we both agreed we should have known better!
Look, ask, talk, and it will often improve things.