It really depends upon why it was considered "used". If it were used for a year, then returned and put in a package - I wouldn't buy it. If it was someone who thought they had a bad PSU, but it turned it that wasn't it and returned it - that would probably be ok. Also - factory refurbished items work well (I have a Dell LCD I bought in 2003 and is still going strong - it was a refurb).
As long as Micro Center offers the factory warranty - and they have a return policy (30-days or more), I would consider it.
There are only two reasons I wouldn't consider it would be if I am building a high-end machine that would be pushing the envelope of performance (i.e. overclocking big time, etc) or the "discount" that is applied to the component isn't "enticing" (i.e. original price is $75 - open box for $70).
Remember - the PSU regulates the voltages to all of your components. If you spend $2,000 on a computer rig, and $25 on a PSU, you put the $2,000 rig at risk. The PSU should be chosen from a Tier 1 (preferred) or Tier 2 (budget) list here: http://www.tomshardware.co.uk/forum/id-2547993/psu-tier-list.html