Almost everything is somebody else's stock
On most retail sites, third-party listings are the exception. Here they are the rule. That means the usual advice — "balance works for the retailer's own stock" — is close to useless.
What actually matters is whether the checkout takes the gift card as a payment method for that listing. In practice this works for a wide range of listings processed through the site's own payment flow, and does not work where a seller is paid another way.
Check at the checkout before setting your heart on a specific item.
Auctions versus buy-it-now
A fixed-price listing is straightforward: add it to the basket and see whether the gift card applies.
An auction is messier, because payment happens after the auction ends and a balance that was available when the bid was placed may have been spent in the meantime. Anyone bidding with gift balance should keep it aside deliberately.
Region
Country-specific, like the rest of the category. The code belongs to one national site and the account has to match.
Part payment is normal
Most orders here are larger than a typical gift card, and splitting between balance and a bank card is the usual case rather than an edge case. See how to split a payment.
Who it suits
Someone who hunts for specific or second-hand things — collectors, people restoring something, anyone after a part that is not sold new any more. Less good for a recipient who wants to walk into a catalogue and pick something easily.
Asking for one
Say the country and the amount. The shopping category guide covers the marketplace question in general.