The thumb screw example would probably be acceptable, since that is a one time thing. Dealers tend to repreatedly sell large quantities of items for their own personal gain. Some of them are selling for other companies, sometimes they are their own company. Most dealers know who and what they are, and sometimes will post regardless of the rules.
Sometimes, if a person is unsure as to whether something might be considered a dealer post, they'll mail the mods to get the go ahead as to whether or not it's ok to post.
If you look at the dealer threads the mods lock, you might get a better idea of what is/is not acceptable.
Mods are there to make judgement calls, so keep that in mind as well. What mods on one forum might deem ok, might not be ok here. Other than what the mods already defined in the rules I posted above, I don't think there is a list hewed in stone of what exactly construes a dealer. The mods sees a post and makes a decision from there.
It might be a good idea to expand that definition, however, though I'm not sure what else would be added. The types of things you're asking are usually dealt with on a case by case basis, where a few might deemed acceptable (such as your screw example) and others are deemed not so.