Just wanted to chime in and corroborate 2NOOB's story; I had the same thing happen to me more than a few years back (5+), so obviously this was prior to the new crackdown "warning".
However, it wasn't a purchase I made on d2jsp, and I paid using real currency. It was a stupid move, I know. Jagex removed all the purchased gold, and sent a warning letter almost verbatim in regards to what 2NOOB received. I'm sure I got caught because they either:
A. caught the seller and bonked all accounts that had activity with them
or
B. the gold was stolen, and it happened to end with me (it was a legitimate site, seemingly, so the person who sold to the site could be the culprit as well, admonishing the site).
Ever since then I've been extremely wary of making any sort of purchase on my account for fear of being banned. That being said, I'm sure it's still statistically safe to continue buying from real individual people, face to face, so long as you employ safe trading tactics and avoid any sort of discussion in-game. Part of me hopes that if I were to continue trading someday, that they would be lenient and only temp-ban if I pinged the system. But.. that feels a bit like hopium so I think I'll just avoid it indefinitely.