my inventory says it has item limit 30/30 but it only has 19 items.

I think it could be items I've made a bid on but when i checked my bids it only had 4 item bids (and 1 pet incase your wondering)

what am I doing wrong?

I keep putting items up for trade but it doesn't make a difference

"I keep putting items up for trade but it doesn't make a difference"

You need to wait for those items to actually sell. As long as they're still in the Trades they still count towards your inventory. 

thank you