  Diffusion auction is an emerging business model where a seller aims to
incentivise buyers in a social network to diffuse the auction information
thereby attracting potential buyers. We focus on designing mechanisms for
multi-unit diffusion auctions. Despite numerous attempts at this problem,
existing mechanisms either fail to be incentive compatible (IC) or achieve only
an unsatisfactory level of social welfare (SW). Here, we propose a novel graph
exploration technique to realise multi-item diffusion auction. This technique
ensures that potential competition among buyers stay ``localised'' so as to
facilitate truthful bidding. Using this technique, we design multi-unit
diffusion auction mechanisms MUDAN and MUDAN-$m$. Both mechanisms satisfy,
among other properties, IC and $1/m$-weak efficiency. We also show that they
achieve optimal social welfare for the class of rewardless diffusion auctions.
While MUDAN addresses the bottleneck case when each buyer demands only a single
item, MUDAN-$m$ handles the more general, multi-demand setting. We further
demonstrate that these mechanisms achieve near-optimal social welfare through
experiments.
