Aaron Hernandez -- Jailhouse I.D. For Sale ... On eBay

Seems Aaron Hernandez got jacked for his official jail I.D. card during his stint in the slammer -- 'cause it just surfaced on eBay ... TMZ Sports has learned.

Law enforcement sources tell us ... the item appears to be a legit identification card from Suffolk County Jail in Mass where Hernandez arrived in July 2014.

Starting bid -- $500.

As for the seller -- that person has not revealed his own identity ... but explained in the eBay post, "I was in jail with Arron Hernandes [sic] the for patriot super star.. I happen to com [sic] across his jail id."

Wonder how that happened?

We reached out to the Suffolk County Sheriff's Department -- but they're not saying jack squat about the situation.