Hey Joff,
Good to hear you're still alive mate!
isnt gumtree a WA thing. I go take a look
Father-in-law bought a bulbar for his car from Gumtree - he saw the ad and range the guy the next day. After a week of ringing around he discovered that he could not buy the brackets for it. Relisted it on Gumtree for the same money he spent, and it sold the next day to a fellow who was terribly excited to get it. You see, he's had these bulbar brackets in his shed for ages...
True story. That's in sunny Cairns, and the bulbar was for an older-model Ford Courier. About as small a market as you could get, yet two sales of the same item within a week, with it being listed for less than a day each time.
tl;dr - give GumTree a go. I was sceptical, but...
I think good, accurate photos go a long way to making potential buyers at ease. Reading your ad, I'm seeing nice pretty photos. But in reading your description "Photos are old ones, trailer is not new, thousands of kms, some red dirt staining, wear marks", I'm starting to think "uh-oh - it's not going to look anything like these photos, sounds like it's had a really hard life, etc...".
From what I know of you I'm confident that you're being far more honest than you need to be, and that everything is actually in quite good condition, you're just not wanting to overpromise and have people disappointed. But Random Joe will be coming from a more cynical position. So I'd suggest trying to find the time to set it up as-is somewhere nice (park, etc, with nothing else around it), and take some accurate up-to-date photos to show the honest condition. I think that would go a long way to making people more comfortable. You're trying to make it as easy as possible for someone to choose your product over the opposition's.
The photos of it behind the cruiser and set up in the bush are gold - you're selling "the dream" as much as you are the camper. And by including them you're showing that you're not trying to pull the wool over anyone's eyes.
Just my unsolicited $0.02 - keep in mind it's worth exactly what you paid for it.
Cheers,
Matto