To add on, mediators can take a lot of time. Your best bet is to expect to go first but be very selective on who you trade with. If it's a user who trades a lot (recently) and has an account they've maintained for years, they likely value their account here and current level of fg more than whatever value stealing a couple of runes from you would bring. If you're trading with an older account that hasn't posted in years, it might not actually be that person using the account. If it's a brand new account, they may have no loyalty/reputation to maintain, and might be more likely to not value their account.
In any case, using the /whois button when they join the game will help, screenshotting/recording things is a fine idea, and just being smart about who you trade with. I would take 5% less than market from someone I felt I could trust in your situation than 10% more than market from someone who gives me an off vibe.