early 14c., "action of defending," shortening of defens (see defense). Spelling alternated between -c- and -s- in Middle English. Sense of "enclosure" is first recorded mid-15c. on notion of "that which serves as a defense." Sense of "dealer in stolen goods" is thieves' slang, first attested c.1700, from notion of such transactions taking place under defense of secrecy. To be figuratively on the fence "uncommitted" is from 1828, perhaps from the notion of spectators at a fight, or a simple literal image: "A man sitting on the top of a fence, can jump down on either side with equal facility." [Bartlett, "Dictionary of Americanisms," 1848].
To establish or restore good relations: I had better get home and mend my fences (1880s+)
A person or place that deals in stolen goods: but even big fences like Alphonso can get stuck/ The loot had disappeared and been handled by a fence (1700+)
: The clown that stole the Mona Lisa found it hard to fence (1610+)
[all senses are shortenings of defence; in the case of criminal act, the notion is probably that of a secure place and trusty person, well defended]