If buy/sell too fast, your items AND plat will disappear. I've been waiting on a petition for this for about 4 months - there's also a thread about it. Sounds cruel and heartless, but hopefully others will experience this as well so that they would make this a higher priority.
Luckily you lost 100pp ish, I lost about 2k (platinum bars). You should bump this thread:
https://www.project1999.com/forums/s...ghlight=vendor