One can simply start a new thread in QSO, or sometimes in Technical as appropriate, titled "Re: (the title of the original thread in sale/trade) and start your discussion from there.
Actually, that wasn't an intended result of making the Classifieds 'author post-only' and is not the way to do it.
Originally the Classifieds allowed replies. IIRC, Gary changed this to put an end to a handful of folks going in to question price, character, etc. of the poster. Anything you need to discuss with a person selling or looking for an item should be done privately - no differently than you would if responding to any other online ad. If you have useful information, simply post it in the appropriate forum. If that info includes 'I have some of those for sale', post it in the Classifieds. Simple.
As Pete says, the policy doesn't prevent starting a thread in the Tech or QSO section to discuss any appropriate topic (ideas for which sometimes come from items for sale or wanted). Just do so in a general way, don't copy and move the topic or posting from the Classifieds section. Moving a subject or posting to another forum to get around the rule defeats the purpose.
For example, someone posts a For Sale ad for XYZ. You're curious since you've heard there are different versions of XYZ. Simply start a new thread in in the appropriate forum along the lines of 'XYZ - How Many Versions?' or 'XYZ - Which Version is Best?'. You could even start the thread by saying something like 'Recently someone was selling XYZ, and it got me to wondering...'.
If you suspect any issues with a posted ad, notify the moderators.