A clarification may be needed:
You need 100 posts to start a thread in the Marketplace. This is for the good of the community, to filter out (in an approximate way) threads by relatively unknown members.
You do not need 100 posts to reply to a thread in the Marketplace, because a buyer or seller who starts a thread is perfectly welcome to buy from or sell to a new member (a "newbie" or "member") who replies. The choice is up to the member whose thread it is.
In particular, members with less than 100 posts are allowed to offer items for sale if they are replying to a "want to buy" thread. We sometimes receive Bad Post Reports from other members in such cases, but these posts are in fact fine under the rules.
Note that if a new member posts about an item they have for sale in reply to a "For Sale" thread, that constitutes thread hijacking, because it is not a reply to the original poster. Those posts should be reported and will be removed.
If you aren't sure about a post, you can always send us a report and the moderators will check it against the rules. But I hope this clarifies the rules about new members selling (or buying) in the Marketplace.