Cross-domain auctions are so **** hard :(.

More thoughts on this: Making a truthful, credible and 2-welfare approximated cross-domain auction is reasonably easy by using DRA (https://arxiv.org/abs/2301.12532) on the bundle, when sellers (validators) do not have access to side-channel/separated auctions.

Note: I think that 2-approximation can be improved by changing the commitments and running a VCG auction. But this is not the problem. When validators can run side-channel auctions, the problem is how to compare the revenue of running two separate auctions to the VCG one.

When there is no cost incurred by bidders that just win one of the blocks, separate auctions are close to efficient (see https://arxiv.org/pdf/1408.1211.pdf). I guess when there are costs, this is no longer true...