In this phpBB software, a person must be logged in to send you an email. If you want to sell to people who aren't members of the Steel Guitar Forum, you will have to include some sort of contact information in your ad.
The "Subject" of your ad should tell the reader what you are selling. If it doesn't, people who aren't interested in it will open the topic out of curiosity. This wastes server bandwidth and annoys people.
Regarding "new" items, the Forum Membership Agreement states:
This rule was created to discourage price wars among vendors. If you bought an item at retail and never used it, you can advertise it as "like new" and post any price you want. But if you say that it's "new", you aren't allowed to post a price lower than the manufacturer's suggested retail price unless you are the sole source of the item.Posting of discount retail prices for new merchandise in the Classified Ads section is prohibited. Readers can be directed to call or email for a discount price instead. Prices may be posted for "used" merchandise.