I too have a standard box with over 9yrs use on mid 300hp but low mileage, been launched & has a tomei shortshift but still A1.
I do believe you thought it was a Quaife as if not posting the pics & coming on here in this thread was pretty stupid.
Although if also had some doubts on what box this was it would of been wise to get it verified by quaife as that effort could of increased sale value by 25% & saved you the current hassle ...
Always a shame to find out what you got ain't what you thought but so many people lie about tuned cars & due to GTiRs cheap price it seems to suffer more in an attempt to boost a sale.
At end of day on a private sale it really down to buyer beware, if you don't know what you looking at get someone who does.
Luckily in this case someone did.
Best bet is class everything as standard unless 100% verified & for engines if not well inspected or with pics & receipts from reputable builder they still ain't worth a lot just cos a set of CPs been slapped in it.
Buying used modded engines & gearboxes is a risky game ... personally I would only buy used standard for parts/casings & build from scratch ...
Purchase wisely & purchase once ...