Tell the guy you realized you made a huge mistake in buying the turbo. But whatever you do, DO NOT INGORE HIS EMAILS. You will get a negative feedback, and possible a banning.
If you explain to him what the problem is, he might take it back. Just tell him to file an unpaid item dispute and check the "buyer and seller have agreed not to go through with transaction" yada yada yada.. he gets the listing fee back and can relist it.
Next time, get a GReddy, Garrett, Turbonetics, Precision, HKS, etc.
XS Power and any of the no name turbos suck.
BTW -- you should be looking for a T3/T4. The turbine AR should be .6 or .7 at the most.. .8 is huge. The compressor should also be .5 or .6 AR.
Good luck