Hey,
the only way to find out whether it's a good idea or not is to give it a try. Everything else is just opinions, and opinions are wrong at least 50% of the time, even expert opinions (well... except for mine of course, I'm always right... no, really... it's true...)
I say give it a shot with this method. Don't charge an upfront fee, but instead make it the way it is now, no-risk for the client. Sure, maybe some people will rip you off, but the majority of people are good people.
Once you have successfully worked with several clients, ask for testimonials from them. Social proof is EXTREMELY important. And if possible, make the testimonials as specific as possible - including ebay user ID, first name, last name, picture, where from, what specifically you helped them achieve. Don't be shy about this. If you can't get people to agree to have all that published, just get as much as you can. (But NEVER make anything up, that's 100% illegal by law, and it will make you miserable in the long run, even if it brings in short-term cash).
How do you plan to get clients?
I agree that a proper website on your own domain is a good idea.
If you don't want to invest a hundred bucks or more, try blinkweb, even their free solution is a lot better than blogspot, and if you get a domain then it's great. It's very easy to create your own professional looking site, specially if what you want is kind of a sales letter. I recommended this one to several people already, and they all love it and having good results.
http://www.omaji.com/blinkweb
Maybe you should advertise locally in your own city - there are lots of people who "know about ebay" and maybe they sell something, and if you offer them that they can get more money for their items and they only have to pay if you deliver great results, than I'm sure you'll find some clients to get started.
Hope that helps,
Johnny Bay