The word «account» in a listing means a file. Files come in three kinds, and not every tool reads every kind.
tdata
A desktop Telegram client folder holding authorisation keys in the application's own format. Fine for opening an account in desktop Telegram and looking at it. For programmatic work it needs converting, and that conversion step is where accounts are most often lost.
.session plus .json
The working format. The .session file is a database holding the auth key; the .json beside it carries connection parameters — api_id, api_hash, device model, app version, language. The second file matters as much as the first: an account created as a phone and continued as a desktop client of another version gets logged out as stolen. See device fingerprint.
So carry the JSON across whole, rather than copying two keys out of it.
Session string
The same auth key folded into one long line. Convenient to pass around, easy to lose: the string is the access, with no password and no second factor. Store it like a key, not like text.
What to ask the seller for
The .session and .json pair. Software reads it directly and it carries device parameters. The other formats either convert into it or are incomplete.
How this works in Neurogram
Upload takes both files at once and reads device parameters out of the JSON automatically — the guide covers the order. More on sessions themselves in sessions explained.
Buy accountsaged, with full session files