I'll try and put this as simply as I can.
When Danny Fyne (Capt Pprune) started this website, it was pretty much a private discussion group for airline pilots, run by Danny in his spare time. Rumours - often wild and inaccurate, but fun - were interspersed with jokes and the occasional factual post. That's where we got the name from.
In the five years since then, Pprune has expanded to become the single most popular aviation site on the internet. It's still run by Danny in his spare time, but its high profile has forced a change of emphasis. Rumours based on fact, and which don't have the potential to damage anyone, are still welcome, but the primary aim of the site now is to exchange information. We now have something over 60 forums for that task, and a couple of humourous ones for those who are into that sort of thing. So the 'rumour' part of our name is now perhaps a little inaccurate, I'll agree - but the name stays!
Wannabes is now the largest single forum on Pprune, and on its own demands huge bandwidth from its server. The servers for Pprune cost some thousands of dollars monthly (they're in the US), all of which Danny is, I believe, personally liable for.
As the membership of Pprune expanded exponentially, Danny recognised the potential for misuse of his site, and put in place a set of terms and conditions of use which you all electronically signed up to. Those terms include the bit about no advertising. Obviously, from the commercial point of view, Danny saw the need to charge for business advertising to offset the costs, so we get upset if a company attempts to get gratis publicity when we'd rather they pay and help the entire Pprune community. Sometimes a company has a legitimate need to post on Pprune, usually to correct wrong information being given out by an individual Ppruner, and that's fine by us. The recent collective effort by some of the UK ground schools to rescue the people left high and dry by PPSC and 4Forces was something of an exceptional situation, and we tolerated posts that in other times we wouldn't. I think the students involved will agree it was the right thing to do.
Now, we come to the personl advert bit. Danny doesn't want you to do it, is the bottom line. The reason is that, once it's allowed to start, it could quickly come to dominate Wannabes, and that's not what we're here for. The task of sifting through, say, a hundred ads a day to sort the commercial from the private (which would most certainly be necessary) doesn't fill me with joy. I've got better things to do with my spare time.
You say that there are other free ad places; indeed there are, but most of them are profit-making publications that sell commercial advertising on the back of the number of readers who want the private ads. As I say, you want to run a site that does that (and can make it pay) and we'll happily link to it. We don't want to run that sort of site, so we choose to maintain our rules. Those rules say that all advertising must be paid for, end of story. We are not going to let you ride the back of these expensive servers to offload your kit. If we could find a way of charging you a percentage to cover our costs, we could consider it, but the logistics are beyond the small group of volunteers that moderate this site, so it's not going to happen.
Yes, a few personal ads have slipped through our net. All of us moderators have jobs and families, and even other interests. We can't be here all the time to catch the recidivists who insist on abusing our hospitality, although we do try.
So I caught you. You broke the rules you signed up to. We're not going to change the rules, and of course we reserve the right to ban people who break our rules. I wouldn't normally do such a thing after one transgression, but I will certainly warn the transgressor that banning is the potential penalty should they be caught again. I'm sorry if you find that unfair, but this isn't a democracy.
As I say, if you want an online aviation mart, go and start one with our blessing. But we're not running one here.