The default 'no price in the title instant delete' is gonna be the easiest to implement.
Will continue to report, but it would be my recommendation to start moderation que and delete any posts without a price in the title. Most modern forums support this function. XenForo says it supports 'Approval queuing'.
I think most come to post things to sell without a price are not normal visitors. I doubt they have read the instructions just like me. Regulars know. I know them from experience. Most will post one when you ask. You can be non tolerant for usually a small infraction. Usually they never come back to check; maybe after a week or two, delete it. There are way worse infraction that stay.
What if I want to put something up for "best offer", or do a "reverse auction"? Is there any reason that something like that should be banned from this forum?
Of all the ban-worthy or warning-worthy things that could happen here, failing to put a price on something is the least important one I can think of.
Advertising a website where sexy Russian girls want to meet YOU !!!, or asking for donations to a political extremist group, or repetitive preaching the gospel of (insert deity name here)... those are big problems worthy of banning someone.
Barter,Sale, Or Free should be standers when posting but in the long run it depends on the person who is posting and it's up to the Forum to get details by PM or other sources. I've asked price and location more then once.