How does the stem look ? online purchased goods that are posted can vary in cost depending on size, therefore smaller items removed from bulky box can reduce postage costs.
Though in your case, surely the stem is what you should focus on to determine if gunuine. You could message seller with regards to packaging in case spec info is on box or leaflet in box etc
I bought a “new” Renthal Apex 35 stem from a reputable seller on eBay. It showed up in a heat sealed clear plastic bag.
I’ve bought one before and it came in a box.
Is it counterfeit ?
Good question- but these things come in a 'display" box rather than a shipping box. I would guess the seller tried to save shipping by eliminating the display box.