Do you PM often when these issues come up?
Part of solution might be to streamline the process. Right now we are allowed to "police ourselves" and have been provided with a polite stock post from Arch to address any guideline issue. Some people notice an item and are hesitant to say anything for a variety of reasons(seller backlash, being labeled "forum nazi", etc.).
Try this: If someone notices an issue and doesn't want to publically/privately(PM) deal with it, send me a PM with the info and I will forward a polite PM to the seller addressing the issue. I get contacted as it is since some people know I will PM a seller about a guideline issue, and the majority I have contacted edited their threads in a timely fashion.
Therein lies one of the other issues. A "timely fashion" might mean immediately for some people, others are patient enough to wait for a response from the seller. I suggest that the PM recipient be given a chance to edit the thread after reading the PM. I know some people get frustrated when the mods are notified about an offending post and nothing immediate happens, and the reality is that it is not a priority for the mods. They do seem to be fair in allowing sellers to modify their posts in a reasonable amount of time or deleting the entire thread if it is not addressed or if there are repeat complaints about the same seller.
1) Funnel guideline issues through specific people to be addressed
2) Notify mods if issue is not resolved. This may mean notifying mods immediately if seller has repeat tendencies. Though we all would like 1st time infractions to be eliminated by people reading the guidelines before posting, I don't know how that can be modified to take place.