The tax free threshold is Y10,000 in a single purchase (at least check). Unlike the inconvenient claim-back systems in most countries, in Japan the tax is deducted then and there, and you're supposed to register the item in question with Customs when you leave. Note that tax-free shopping is very much the exception in Japan, not the rule, but most tourist-oriented shops and the large electronics superstores in Akihabara/Shinjuku can handle it.

I would forget about trying to make money leveraging goods you don't much care about, Japan is just too darn expensive for that.