As a new seller, you usually don’t have a portfolio. I’ve seen some people showing fake portfolio from anywhere around the web to gain buyer’s trust.
Only show the portfolio if your name is present on the website somewhere. But it’s a better practice to have a portfolio URL in your Gig or show it in inbox while discussing (only if the buyer asks for it).
Side Note: Try to include a Flickr link in the Gig or profile. Not your own website etc.
Don’t make your proposal scream “I’m hungry for your Money & getting orders!” instead show that you genuinely want to help them. Don’t show fake portfolio, instead let them know that you have to knowledge required for the task.
A Sample Buyer Request Proposal:
“Hey! I would like to get this task done for you and I’m available to do it right now for you. I have understood your requirements and have the required knowledge for this.”