There are lots of free forums around. phpBB is one of the free
ones and most popular ones.
But being popular means it's more prone to being hacked. Being
the most popular also means it has lots of modifications to suit
your needs (skins, settings, etc)
On the server issues, you have to ask yourself what you
need.
Since you're selling things... a few to consider.
1. Bandwidth. Most likely you will be putting up pictures of
your products. You will need lots of bandwidth.
2. OS. If you choose open-sourced solutions as your forum
(phpBB, PunBB, YaBB, etc), you will most likely use an open-sourced
OS and an open-sourced server (Apache).
You can still use Windows, but it needs some configuration. Not
recommended if the ISP doesn't want to bother with that. Most web
masters configure their own servers.
3. Uptime and downtime. Even with no 100% uptime guarantine, you
should at least expect a 99% uptime. Uptime meaning it's running
without issues.
4. Backup. Either is you do it or the ISP do it.
5. Price. LOL... this is obvious.
6. Location of server. If you are targetting locals and want
them to have fast access, a local ISP is preferred, rather than an
international ISP.
The Queen of IT has just spoken. 