I run the TandemClassifieds.com website, so I want to respond to this.

1. Several months ago (mid-2015) I noticed a couple of ads that were suspicious, and one person told me about another suspicious ad. I deleted the ads promptly and added a warning to the top of the website home page.
2. Since then I have done some checking into each new ad request. When I started doing that I found a couple of suspicious ads. Since then I have not found any, but I continue to check.
3. I agree with the words "I would be very careful.", just as I would agree to be careful about any long-distance purchase. Consider using PayPal. Consider getting a reference from a local dealer. There are other good suggestions elsewhere in this forum and online.
4. I feel that the words "...that site has been a target of scammers." overstate the problem, since the problems occured several months ago, and I have addressed them.

I frequently get complimentary emails from people who have successfully sold or bought a bike that was advertised on the website. The website is certainly not perfect, but it is a service that I provide for the tandem community simply because I enjoy tandems.