You can always search "from outside" the forum, such as by Google (except it won't find things posted subsequent to Google's last crawl through the forum).
If you want to search the buy, sell and swap subforum type your search followed by the subforum web address. Like:
semi auto A cat sten gun $450 Buy, Sell or Swap
...and google should return results from within that.
As for editing posts, I'm with you, but we just have to learn to get it right from the outset, or at least within the first half hour. Not fair on those who reply to the posting if their reply is to something you later edit out. Gets weird and more frustrating and may create more bother than it solves. Personally, posts should not be editable (though should be deletable in the first 30mins), but the permanent ability of the original poster to insert footnotes seems a good idea.
Like the old confessions of the cultural revolution, plenty tears and hand wringing:
"I reflect on posting, it was disgrace, I repent, I let down Communist Party of glorious People's Republic. I aks your forgiveness my reactionary attitudes. My bad thought delay glorious victory of People's Republic over capitalists who buy glorious exports make us rich. I aks forgiveness, glorious forum in humble footnote at your feet."
Bookmarks