I often have items that I am searching for, but they seem to be sold as soon as they pop up. This can be very frustrating, especially if it was a screamin deal. This is why I use google alerts. Google alerts is a way that I keep up with any posts for a very specific item.
For example, say I am looking for a Zeiss LRF, I would go to google alerts and type in the parameters that I would like in the query. Now google alerts has many benefits, but it also has a downside. The benefits are that you can get hits for "zeiss lrf" sent to you in real time. You can get these notices sent to your email, and with smart phones this means you can look at the item right away.
The downside is, you are going to be notified any time ANYONE posts about a zeiss lrf. But this is why you should be very specific in what you are searching for, if you just searched for zeiss, your inbox would be flooded quickly. But a search for "WTS Zeiss LRF" would probably get you more the results that you were looking for. I usually use google alerts for very unique items I am looking for like a "6.5x47" or "Dan Dowling custom". This keeps the emails to a minimum, but still produces fairly high quality results. ENJOY!