Your safest and smartest path:
However, if you already own GW2, ask in the GW2 community forums. Occasionally, veteran players who bought extra keys during the "Nostalgia Sales" give them away to new players in exchange for in-game gold or as a random act of kindness. This is rare, but legitimate.
Historically, Guild Wars had a "Refer a Friend" system where existing players could generate trial keys for new players. If a website claims to have leftover trial keys, they are almost certainly expired or already used.
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