I do a lot of buying/selling guns on a couple of website up here in Canuckistan. My ads are very clear:
"Payment via EMT within 24 hours of agreement to purchase"
I live quite rural, and don't intend to wait for the post office to deliver a check that I then need to go into town to cash and then wait for the funds to clear. We're a quarter of the way through the 21st century; if a technoplegic like me can figure out and use EMT (Electronic Money Transfer) then anybody can.
The two websites on which I buy and sell don't even allow a potential buyer to post in a For Sale thread. You contact the buyer/seller to make an offer; once he responds and a deal is struck, the buyer sends the funds and then the seller ships the item. Yep, there needs to be a certain level of trust; I have had two problems (out of over 600 transactions over almost 30 years), both of which were eventually resolved. I make extensive use of the Trader Rating system on those sites; I will sell to an unknown individual but will only buy from another established member with multiple positive ratings.
Up here in the Great White North, our lizard overlords require any gun owner to have a licence (a PAL...Possession & Acquisition Licence) that must be confirmed before a firearm is sold. As much as I hate it...I will not even consider sending a gun to someone unless we do all the legally-required red tape BS. I have too much to lose, and the process takes maybe a couple of minutes online.
Buying and selling is easy if you accept that no individual transaction is an absolute must; plenty of attractive items come and go without my even trying to buy them, simply because I didn't feel comfortable dealing with a complete unknown quantity. And plenty of my ads sit for a few days or weeks waiting for somebody who wants to cross all the T's and dot all the I's. I don't need to sell these things to put food on the table; I can wait for a legitimate buyer.
If a buyer says "I'll take it!" and I then say "Sold!"...it's sold, and the ad is marked SPF (Sold Pending Funds). Nobody else need apply, even if they want to offer more money. But, again, the ad specifically states "payment within 24 hours"; if that period expires without payment or at least an explanation, then I either move on to another buyer who may have asked me to contact him or I just edit the ad back to For Sale.
Communication is critical; someone who just stops communicating, without an explanation, isn't worth my time. I provide updates on shipping, tracking, etc. all throughout the process, and I submit a Trader Rating as soon as I receive payment when I'm selling, or as soon as I receive the package when I'm buying.
Edited to add: One last thing: when the item is Sold and shipped out...for pete's sake, please take the time to indicate this in the ad, or delete the ad if you are allowed to do so yourself.