The Mogen Avraham (306:15) quotes the Yam Shel Shlomo
(Maharshal) who was hesitant with the practice of selling on
Shabbos/ Yom Tov , Mitzvohs to the highest bidder, being
it looks like buying & selling.
The Maharshal continues, he prefers the practice of the
חסידי אסטרייך , all the bidders pay the amount they bid even
if they don't end up winning the bid.
This looks more like pledging for Tzedaka than buying &selling.