In regards to a point systembuilder (the OP) brought up & jetter responded to, the only way buyers & sellers are officially allowed to communicate directly is if the seller has paid 4% of their asking price upfront for a 30 day ad (Regardless of whatever price it eventually sells at - or if it sells at all) & is non refundable.
If one wishes to communicate with the buyer/seller directly w/o an upfront payment, the only option is to include your contact info in a free (the only kind offered), US Audio Mart ad (that can run indefinitely) placed simultaneously. It’s fairly common for AG ads to be simultaneously put in US Audio Mart & continue to run after the AG ad has expired. I don’t know how often contact info is also there, but I’ve certainly seen it. Letting the buyer know directly or indirectly i.e. asking them to google the particular product being sold on AG, sounds like it could possibly be problematic, but I have not the personal experience or know others who’ve done so to comment. There’s certainly no strictures regarding mentioning the competition on AG forums, I’ve found so far.