20% is bloody cheap. SAFC (new) yahoo prices are 27,900yen - so he's doing it for pretty much nothing (in my opinion). I charge a flat rate of 10,000 for just about anything I do (multiple units, okay) - not worth my time and effort otherwise. If he's charging you 20% on a 2nd hand unit - he's crazy (which is good for you).
The domestic freight is going to be around 1000yen or so.
Not sure of the weight - but, as an example, a 2kg package is going to cost around 4,500yen via standard airmail.
and, of course, there's always the chance of getting pinged for duty when it arrives...