Add POST ICONS for each item.
Have the user choose the correct post icon as it relates to their post.
I also have seen colored arrows used.
RED - For Sale
BLUE - Want To Buy
GREEN - Group Buy
But you already have forum categories with sub categories.
There is nothing wrong with having your users place their posts in to the correct forum. Having moderators will help that process happen. They can move the posts to the correct categories. edit them. correct the prefix. inform the members that posts related GOOGLE DEVICES go in to the ANDROID FORUM (or should use a green robot post image). and posts related to anything iOS/iPadOS go in to the APPLE FORUM (or use a rainbow apple post image). Having correctly titled categories not only help your users, but also make the jobs of your moderators much easier.
If you plan on having a large forum, you should also plan on having moderators.
