I am a bit skeptical of the "keep bids below a certain amount" approach, the moment a limit is given, the incentive to go ever so slightly above the limit to claim and item would be quite pernicious (though the higher the limit the less ideal such a move becomes), whereas with no ceiling but a definitive bidder for each item gives someone no idea of how they could "steal" an item, and thus they would be less likely to even try, ensuring at least $2000+whatever the assigned bidders don't give to go to the aquariam, and if less than 2000 went in, or someone didn't get the item assigned, well, the culprit would be easier to find than in the assigned limit case, as a group of 4 is a lot smaller than a group of 10.
Though of course, 4000 in the pot is twice 2000.