With an existing app, login control fails before the app does
Apps, certificates and Bundle IDs all sit under the Team, and the only way into the Team is the Apple ID. The usual failure after a handover is not a missing app. It is a login that cannot complete: the verification code still goes to the old phone number, the trusted device is in the seller's hands, or the recovery email is unchanged. Ask who controls the sign-in chain before you look at the app list.
Verify three things on the Apple ID: email ownership, recovery, and whether it is a dedicated ID
First, confirm you receive full control of the sign-in email, including the mail provider console and not just the password. Second, confirm the recovery email and any recovery key or security details are replaced too, otherwise the previous holder keeps a recovery path. Third, check whether the ID is used only for the developer account. If it is also tied to the seller's iCloud, purchases or Family Sharing, the handover drags personal data along. Ask for a dedicated ID, or walk away.
Two-factor decides who can really log in, so replace both phone number and devices
With two-factor enabled, a new sign-in needs a code from a trusted device or trusted phone number. Check two things: the trusted number is replaced with one you will keep long term, so codes no longer reach the seller; and your own device is in the trusted list while the seller's is removed. Turning two-factor off is generally not available under current flows and should not be part of any handover plan. If the seller says they will forward codes whenever needed, control has not moved. Treat that as a stop condition.
Device binding is not account binding, but it affects certificates and releases
The developer account is not tied to one computer, yet signing certificate private keys live in the keychain of the Mac that created them. Without the private key you can revoke and reissue certificates, but you must check how that affects signing, push certificates and provisioning profiles for the existing app. Registered test device UDIDs also count against the yearly quota, so confirm the count and whether they stay. Put these in the handover list up front.
An app on the account must be cleanly tied to the Team
Go through App Store Connect item by item: is the app live, removed, or only a record; does the Bundle ID match what you plan to use; are there pending reviews, TestFlight builds, subscriptions or in-app purchases. Paid apps agreements, tax and banking details are account data as well, so settle whether they transfer and who updates them before you order. Exact fields and flows follow Apple's console at the time.
When to proceed and when to stop
Proceed if the sign-in email, recovery email, phone number and trusted devices all move to you, and you can complete a full login and open the console yourself during the handover window. Stop if the seller wants to keep code forwarding, refuses to change recovery methods, mixes personal data into the Apple ID, or cannot explain where the app came from. Account type (individual or organization), country and whether an app is included all change the required documents, so match each against your order rather than comparing price alone.