there are two reasons for that: one, bmsbattery is the bellwether - their 09 sinewave controller, display, PAS, brake levers and cable harness kit is of similar specs sells for $85. Wholesale trade price for the kit is about $75. Add to that kit a BPM motor wheel and 13AH 40 cell battery - you come to about $500 - that's the price ebay traders pay. The difference between a bmsbattery of similar specs and the oxygen is mainly cosmetic. Two, you'll need a good donor bike to justify spending £630 on a kit. Add the two together, you'll be spending north of £950. Few self builders will want to spend that much money on a bike that is essentially a looker rather than a beast, ergo the number of potential buyers are only tiny few.