Tacit negotiations can work. And in practice, it's often your representatives that do the negotiations with other people's representatives.
See https://en.wikipedia.org/wiki/Logrolling
>> or who play repeatedly. > I don't see why I should expect that to make the problem easier.
Check out the repeated Prisoner's Dilemma for some inspiration for how repeated play can breed cooperation.
> That sounds like it could go incredibly wrong. Everyone who is willing will sell themselves out to the highest "bidder" (maybe bidding via money, maybe promises of future laws...), [...]
How is that different from people selling their vote today?
Just make sure that the legal system does not enforce these contracts, and you are good. (You can also make such contracts illegal completely, just like selling your vote today is illegal in many countries.)
> [...] and the population unwilling or unable to become a member of parliament will have no say in the matter.
You can cook up slightly more complicated versions: every voter nominates a (willing) candidate on their ballot. Nationwide, you collect 600 ballots and fill up parliament with the people named on them. Pick your favourite resolution method, in case the same person gets picked multiple times in your sample.
(Eg you could give that person more weight in parliament, or you could pick the voter's second choice, or you could pick the ballot of the guy who got picked twice to pick a replacement, etc.)