Delegated voting is just a way to vote on decisions, it doesn't say how those decisions get put in front of the system itself. E.g. they could still be made by politicians or a Swiss-style system in which a number of signatures must be gathered.
I'd always imagined that the way you voted is visible only to your delegees. Thus buying votes at scale doesn't work because to verify someone voted the way they said they would, you need another voter to delegate to them. You gain visibility into one vote but it costs you one vote, so there's no benefit to be had by attempting to purchase. Of course you can try and purchase the votes of trusted figures to whom many people have delegated their vote, but this is no different to the existing system where you could try and bribe MPs.