From Merriam-Webster Online Dictionary:

Main Entry: 1sign
Pronunciation: 'sIn
Function: noun
Etymology: Middle English signe, from Old French, from Latin signum mark, token, sign, image, seal; perhaps akin to Latin secare to cut -- more at SAW
1 a : a motion or gesture by which a thought is expressed or a command or wish made known b : SIGNAL 2a c : a fundamental linguistic unit that designates an object or relation or has a purely syntactic function d : one of a set of gestures used to represent language; also : SIGN LANGUAGE
2 : a mark having a conventional meaning and used in place of words or to represent a complex notion
3 : one of the 12 divisions of the zodiac
4 a (1) : a character (as a flat or sharp) used in musical notation (2) : SEGNO b : a character (as ÷) indicating a mathematical operation; also : one of two characters + and - that form part of the symbol of a number and characterize it as positive or negative
5 a : a display (as a lettered board or a configuration of neon tubing) used to identify or advertise a place of business or a product b : a posted command, warning, or direction c : SIGNBOARD
6 a : something material or external that stands for or signifies something spiritual b : something indicating the presence or existence of something else <signs of success> <a sign of the times> c : PRESAGE, PORTENT <signs of an early spring> d : an objective evidence of plant or animal disease
7 plural usually sign : traces of a usually wild animal <red fox sign>
- signed adjective

The seller must assume that #5 fits his needs to use advertising pages from magazines as a "sign".