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All answers having "Buy" as clue:

3 Character Answers:

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GET



4 Character Answers:

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SHOP



5 Character Answers:

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ENTER

TRADE



6 Character Answers:

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ACCEPT

PAYFOR

PAYOFF



7 Character Answers:

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BELIEVE

FALLFOR



8 Character Answers:

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PURCHASE



12 Character Answers:

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DOUBLEAGENTS