Give every physical item one permanent SKU. Write that SKU on its bag or tag, enter it in one inventory row, and put the same value in eBay's Custom label and Poshmark's private SKU field. Keep the storage bin in a separate column. That lets you move a jacket from B-06 to C-02 without changing its identity or editing two listings.

The minimum useful row is SKU | item | purchase date | cost | bin | eBay URL | Poshmark URL | status. Add listing date and asking prices if you review aging stock. The free Reseller CFO inventory and P&L tracker includes a stable preprinted SKU, item, source, cost, bin, listing date, platform, status, and days held. It is a manual tracker; you still update listings on the marketplaces yourself.

Set up the physical side first

Label shelves or bins with short, unambiguous codes such as A-01 and A-02. Put a matching item tag inside each bag. Record the SKU and bin when you put the item away, while the item is in your hand. A bin-only system breaks when stock moves; a SKU-only system tells you what sold but not where to find it. Poshmark's own inventory guide describes numbered bins paired with a spreadsheet and suggests recording cost, bin, and listing date.

Keep the SKU short and stable. INV-0042 is enough; it does not need category or location encoded into it. Those properties can change. Avoid using the spreadsheet row number as an item ID unless it is stored as fixed text, because sorting a formula-generated row number can silently relabel the item. The Reseller CFO workbook stores its INV-0001-style values as fixed text for that reason.

Put that ID on both marketplaces

eBay's Custom label (SKU) is an optional seller-only inventory reference. Poshmark's private listing fields include a seller-only SKU and cost price. Enter the same SKU on both listings, then store each listing link in your inventory record if you want one-click access during a sale or price review.

Example: one jacket is INV-0042, stored in B-06, listed on both sites. Poshmark sells it. Find INV-0042 on the tag, remove the eBay listing, mark the inventory row sold, and log one sale against INV-0042. The workbook can pull the jacket's original cost into the Sales Log. It cannot automatically delist the eBay copy, so make that step part of the packing routine.

Audit the system once a month

Pick a bin and reconcile every tagged item to the spreadsheet. Then compare your active listings with the sheet. Poshmark's Inventory Report includes SKU and listing dates for available listings; eBay Seller Hub supports listing and report downloads. Fix missing SKUs, stale statuses, and duplicate platform counts before using the data for sell-through. One item listed twice remains one unit in your store sell-through calculation.

If manual delisting keeps producing double-sale risk, test crosslisting software against that exact problem. Preserve your SKU and cost export so a tool change does not erase the history you need for margin and inventory decisions.