It depends. There are pros and cons. Being scammed is something that I think you can avoid with the feedback options ebay has.

However, the more important thing is that you buy a bike that you may not have ever seen or sat on. That may be a good idea if you know exactly what you want. If that is not the case, then you are better off seeing a bike in real life. Especially if you are a first time buyer. Most of the time people would not even be sure what size frame they need.

In your case, if you have a specific frame in mind and already know a few things about that frame, I think it would be fine.