WholeCell allows you to connect a single SKU to many marketplace listings. This is a powerful feature, particularly with mobile devices, and used products of all kinds. For example, a GSM-Unlocked cellphone can be sold on multiple carrier-specific (AT&T, Verizon, T-Mobile, etc) listings. Also a used product in "B Grade" cosmetic condition might be acceptable to sell under multiple conditions defined by a marketplace.
Cross-listing provides increased exposure and typically faster sales, but it also has caveats. It can lead to overselling especially when listings are at low quantities and are selling fast.
If you have qty:1 of an item that is listed on 4 separate listings on the same marketplace, WholeCell can't prevent oversells if orders are placed for multiple of those listings within WholeCell's 10-20 minute qty sync interval.
Additionally, WholeCell uses marketplace APIs to send quantity updates and sometimes these updates are rejected or fail silently. To handle this redundantly, WholeCell re-checks listing quantities once per hour and then fixes any quantities that are out-of-sync on its next 10-20m sync interval. In these rare cases, a qty might not get updated for multiple sync intervals.
WholeCell does its best to keep your inventory quantities in-sync, but it is not a silver bullet. No system can be. All marketplaces experience outages and occasional API problems.
It's best to think about the possibility of multi-channel overselling as a spectrum. Whenever you list multi-channel, you've entered into a non-zero possibility that you will oversell. The more that you cross-list between channels, that chance goes up. The more that you cross-list on the same channel, the chance goes up. The lower your prices (high likelihood to sell fast), the chance goes up. etc etc.
As cross-selling best practices, we generally recommend that you:
Keep a reserve (unlisted) qty on-hand for high-demand products
Limit cross-listing of SKUs that you are not able to re-order easily
Balance exposure between high and low bandwidth marketplaces
Stay aware of your SKU exposure when setting listing prices