If participants are not forced to submit bids for selected lots, there are chances that they might unintentionally miss out on the ones they wanted to bid.
The select lots(Step 3) option that is provided to participants to let them choose specific lots that they want to submit bids for. If a participant does not want to participate in a particular lot they should uncheck that lot and proceed to submit bids (Step 4) and this should be handled through training. Have you considered Staggered lot opening instead of Parallel for "
Alternatively, when you create your content, as a event owner, you can choose to invite only specific participants at lot level i.e. if you have 3 lots and 6 participants for the whole event, you can specify that in lot 2 you want only 4 participants.