Fix the premise first: apps can be transferred, the membership has no official transfer flow
Apple's Developer Program License Agreement restricts transferring membership, and the exact wording is whatever Apple currently publishes. What the platform does offer is App Transfer, which moves a single app between teams. So the question of what still works after the deal splits in two: either login control and contact details change hands while the app stays in its original team, or the app is moved into your own account through App Transfer. TestFlight and listing behavior differ completely between these, so ask which one the seller means before you pay.
TestFlight: old builds depend on expiry, new builds depend on upload access
TestFlight builds expire after 90 days, after which testers can no longer install them. After a handover, builds still inside that window usually stay installable, but continuing to test means uploading new ones yourself. Internal testers must be App Store Connect users, so any role change alters that list. External testers join by email or public link, and the first build of a new version typically goes through Beta App Review. A practical test: ask for a screenshot of the build list with expiry dates, then confirm you can upload and distribute one fresh build after handover.
Live listing: downloads don't need your login, updates and reviews do
An app that is already on the store stays downloadable whether or not the new owner has logged in. What depends on the account is everything that comes next: submitting versions, answering App Review, changing pricing, handling removal notices. The app record is tied to its Bundle ID and Team ID, and the seller name shown on the store comes from the account entity. If an agreement is unaccepted or expired, submissions and paid features can be restricted, with the exact effect shown in the portal. Check three things: current app status, the latest review history, and any pending items under agreements, tax and banking.
Team-bound services are what people forget when an app changes teams
With App Transfer the app lands under a new Team ID, so anything tied to team identity needs rechecking: push certificates or keys, Sign in with Apple, App Groups, iCloud containers, the team prefix in Associated Domains, and in-app purchase or subscription setup. Apple sets prerequisites for transferring apps with certain capabilities, and the checklist shown during transfer is the authority. If the account is simply handed over and the app stays put, these settings don't change, but risk moves to account permissions and certificate ownership. Either way, ask the developers whether the code hardcodes a Team ID or an old certificate.
Pre-order checklist: ask for verifiable portal state
Go through these items one by one: individual or organization account, and whether membership is active; how Account Holder control is handed over; the status of each app; TestFlight expiry dates and tester sources; any pending agreement, tax or banking items; who holds certificates, profiles and push keys; and whether any review warnings or violation notices exist. Fees, terms and review states change, so rely on the platform's pages at that moment and on a written agreement, not verbal assurances.
When to stop or pick a different route
Stop if the seller can't prove Account Holder control or show the real portal state. Stop if an app carries unresolved violation or removal notices, because those transfer to you along with the account. If you only want ownership of one app, official App Transfer is the cleaner path than buying a whole account. If you only need an aged entity and don't want history, an aged account without apps is simpler. After handover, run a real check within the agreed window: upload a new build, start an internal test, and submit a minor version if needed, then settle once the pipeline works.