I have bought 4 bikes from interstate in the last few years, ranging from 2k to 6k. 3 of them shipped to me unseen apart from pictures, one of them I did a 1 day 1300km round trip to Bacchus Marsh from Adelaide, the seller drove out from east Melbourne to save me doing the city crossing, all were found on Facebook pages so checking out the seller is a bit easier than a scumtree buy.
I'll send a few messages then when it looks like a goer, I'll have a chat on the phone. On 2 occasions I have sent the seller a friend request and even screenshotted some family members. Also ask when the pictures for the sale were taken, sometimes they are 6 months old when the bike was new.
There's a decent amount of trust involved to send 5k interstate for a carbon bike you have never seen but FB is a (little bit) safer platform as it's not as anonymous as the other options.
All FB buys have been 100% with no issues ever.... Not so much for the meth head who stole his dads PayPal account and went on a 40K spending spree in Adelaide, I lost a $1000 laptop but there were people who lost gear up to 4-5k, expensive jewelry, Petrol Go Karts, secondhand dirtbikes ect.... PayPal put my account in arrears of -$1000 and told me I had 30 days to make it right or it would be sent to a credit collection agency