Guild Wars doesn't have a subscription, but you do need a unique access code to be able to register and play the game. Without that code, you will not be able to create an account and you will not be able to login and play the game. If you buy the box second-hand, that code will already be used (by the guy who bought the game new)... meaning you basically just pay for the box contents, without getting the ability to play the game.
Note that the box itself is useless... you don't need the installation cds, as you can download the guild wars client for free from their website. All you need to play the game, is that (new and unused) access code.
Also, keep in mind that selling/buying of used accounts is against the ArenaNet End User Agreement. In other words: if you should be able to successfully buy a second-hand (used) account and have the original owner transfer their entire account to you, then if ArenaNet finds out, you run the risk of that account being closed and you being banned. Plus, you never know what illegal activities that account has already been used for by the previous owner, so if that owner has been involved in any activities that are not allowed in the game (for example botting or gold selling), then that could be another reason for the account being closed/banned at any time.
In short: never buy a used account, always buy guild wars accounts new. Right now, if I search on Amazon UK for Guild Wars ('new'), I see that you can already buy Prophecies (the original 'Guild Wars' campaign) for £5.99, Factions (the second campaign) for £2.99, Nightfall (the third campaign) for £5.00, and Eye of the North (the final expansion) for £4.89. Which means that for less than £20 you would have the full game brand new and unused, so why buy a used box at all?