As someone who currently runs a Canadian domaining forum I can tell you that would be a bad idea.
I had extended edit times and eventually people hid malware links after the post had been proof read by me or a moderator. Eventually I banned the person but he rejoined under a different name and then hid the links in the pictures of posts.
I never thought to click on the pictures to see where the links went.
Eventually I changed edit times to 1 hour all the way across the board including sales topics. The hiding of links was starting to take hours of my time every day. Now I only have to check posts once because they lock after 1 hour and as long as I proof read them after an hour has expired I know no harm can be done.
As far as For Sale topics I tell my members to post corrections underneath.. ie... reduced or sold etc.
Quite honestly I prefer that because it is way more transparent with everything recorded instead of people going back and deleting and throwing the topic way of base.
Just my opinion on this because I have other things to do with my day than rereading dozens of posts when people try to do malicious things.
We will never extend it out that long, but will act as a medium if you do want to edit something in that keeps the posts in context, at least when it comes to discussions. This is because editing (or deleting) posts in threads can harm the flow of conversation that took place over those days and make it read out of context to just be plain jibberish to future readers.
The marketplace is another topic. We might extend it out to a week or so in order to edit the first post to always include new information, etc.