I currently run the gigabyte windforce oc r9 280 on the evga 500b, it runs fine, even under long gaming sessions, although I don't have any crazy accessories in my case and no OC.
The psu you chose would run it but I wouldn't recommend it as it may not produce enough power under load sometimes and force your computer to shut down.
I would recommend not skimping out on your power supply, look for like an XFX 550w gold or something of that nature, the evga b2 and g2 series are great too but may be 'overkill' as they start at 750w but they're really good value for performance.
About buying used parts, I've never done it before but I've certainly considered it, the things you probably have to watch out for is if the card has been used for mining. Shouldn't be really any much of a risk and if it helps you get a better psu I'd say go for it, unless the price seems 'to good to be true' then it's probably fked.. Maybe I'm just being skeptical.
There are a lot of great bargains online
http://uk.pcpartpicker.com/part/xfx-power-supply-p1550sxxb9
Something like that would suffice for a 280x and still leaves a little bit of headroom