Bartering is a great way to learn the business, industry and build your network. I would recommend bartering with people that can give you value such as a business coach, established va's etc. This way you learn about building your business at no charge. For example:
A million dollar coach that teaches you how to build your business to a six figure income and they charge $1000 for their program or $200/hr for consulting services. You ask them to barter the program or consulting for no charge VA hours. it's a win win situation!

Once the barter is over and you have successfully helped the coach chances are they will consider you for more work in the future or refer you to someone that needs a VA.
Another option is to barter VA hours for referrals. So find a internet marketer or business coach that is well known. Ask them to refer and promote you as a VA and give them free VA hours in exchange.