Nah, it's basically 4 am here so I could have misunderstood as well
No, doesn't need to be empty.
Like I said, to avoid getting that error keep your inventory below 50 items (the max. allowed) if you have 50 or 50+ you won't be able to buy anything either from main shops or user shops. That's why Stealth Buyer Pro sends everything to your SDB (?, not 100% sure) when you click start... But sometimes it does, sometimes it doesn't. (90% of the time it doesn't do anything for me. Haven't really tried to find the exact reason though/reproduce the steps e__e)