eBay refills for prepays ?

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http://forums.androidcentral.com – I realize this isn't strictly Nexus related, but since I have one on the $30 prepaid T-mobile plan, I figured I asked here. I'd never considered doing this, but I saw it being discussed in another thread and it got me curious. I'm assuming that T-Mobile isn't gonna tack an additional 6% tax on when I go to apply this, correct? (probably soon to be a moot point given the hoopla about making eBay charge tax). More importantly, how "safe" is doing this? For whatever reason, I'd always assumed that selling gift and payment cards and such on eBay was potentially a scam, so I haven' (HowTos)