sorry for the late reply buuuuut here's my advice.
most of the irl dealers are going to have a high rank. be extra wary if someone says they'll buy you something and they don't have a rank/have a low rank.
like kvids says, the irl dealer always goes first. he explained that one well
take screenshots of everything. whether you're planning out the deal in game, on discord, whatever, make sure the screenshots aren't cropped and show the chat. if it's a higher ranked person, use /realname to find out their ign so you have proof of that. most of the og comets on skyblock are super friendly, so you can message a few of them and say "hey, i'm doing an irl with so and so, do you know if they're trustworthy?" if you feel suspicious that they're gonna scam you.
you said you're going for astro rank. if no one's explained the economy to you in game yet, for most irl deals, irl dealers go 1mil+ for 1usd. when i did irl deals i only did ones where people were overpaying (for example, i'd buy someone astro for 4-5mil)
once you get the money, just say in chat "paying (x amount of money) for astro rank, pm me!" or something like that. if there are any irl dealers online, they'll message you
just be careful, take screenshots of everything. good luck :')