If people want the privilege of viewing trust, all they have to do is sign up for a free account. Seems reasonable.
It is reasonable. But if the forum can show trust and avoid a few scams then it definitely should, especially considering guests don't even know they can have that privilege.
if someone is, we'll say gullible, to the point where they are willing to trade with someone here before signing up and learning how things work. Is there really anything you can do for them?
People don't need to be extremely gullible. They may not know bitcoin is not reversible or they could think Selly has some basic buyer's protection and can reverse the payment (like PayPal) if they're used to eBay for example. They do need to be naive but not stupid. If the forum can help those naive people then it should. Showing known scammers' profiles as red would certainly help them.
And, whatever the reason, this happens and users are being scammed by known scammers.
See some posts here for example. Users have been scammed and wished they knew about the trust system sooner:
https://bitcointalk.org/index.php?topic=3035620.msg48843643#msg48843643Points to your own trust, Sandy, would you be fine with your relatives or potential employer reading the negative troll feedback left on your account if they did a google search for you? If they don't have any context about the Bitcoin forum, its probably far more negative from their perspective than the people who know to ignore it here.
It would make sense to show default trust, not every trust left by anyone. The forum could treat guests as newbies who don't have a custom trust list.
Anyway, theymos has already replied to this,
as noted by actmyname, so I guess that's decided unless this current trust system fails and he implements the "force-custom-lists solution".