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Bikey customers burn up builders' time and energy on an already margin slim product - the longer they are in the system the more they will burn up your time, your margin. Then you add the cost of the basic management of the sheet. Then you get to make exciting choices like when do you order the tubes? Up front so as to get a volume discount, or further down the line so as to allow changes. But where to store the tubes that you aren't building with? Do you just build the thing then hold on to it for a bit? What if it gets damaged whilst in storage? There's other bonkers stuff you'd need to do such as if you want to work out how much it's actually costing you to deliver a unit you'd need to do it across the different payment schedules. No idea how you'd account for it / VAT it either assuming VAT is a thing on bike frames.
Basically it's a complexity explosion that's already been mitigated by the advent of the credit card.
It's easy if you don't actually want to run a profitable business, yes :)