Trading up works because most traders are impatient. Your advantage is being willing to take a 5% win twenty times instead of hunting one enormous win that never arrives.
Stage one: volume
Start with high-demand low-value items. They trade constantly, so you get many chances to take a small edge. Never chase value at this stage.
Stage two: consolidation
Once you hold five or six mid-value items, combine them into one better item. Consolidating is where an inventory actually grows, because one strong item attracts far better offers than a pile of weak ones.
Stage three: patience
At the top end trades are rarer and slower. Post a clear ad, wait, and refuse every lowball. One good trade a week beats five bad ones.