Why is selling aliyos allowed on Shabbos and yom tov?
Pledging a specific amount of money for non-mitzva purposes is prohibited on Shabbos under the rubric of daber davar (discussing mundane matters). However, when pledging money by a mi shebeirach, the Rama and Alter Rebbe record the accepted minhag to allow stating a specific amount, as it is permitted to pledge tzedaka on Shabbos.
However, poskim question the permissibility of auctioning aliyos to the highest bidder, which resembles a bona fide sale. Mekach umemkar (buying and selling) is prohibited miderabanan on Shabbos, and it is not allowed even for mitzva purposes. While they justify the practice by noting that no tangible object is being sold, they advise a yerei shamayim to donate his bid to tzedaka, regardless of whether he wins the auction. In this way, it is essentially a tzedaka pledge and not a sale.
Some communities are particular not to mention the currency, just the number or multiples of chai, to make it noticeably different.
Yet, the accepted custom is to sell aliyos even with mention of currency, and poskim write that the minhag of Yidden is itself Torah.
Additionally, the bidding expresses the honor of Torah on Simchas Torah and Shabbos Bereishis.
An auction for tangible items, such as seats in shul, is not allowed according to all opinions (unless all bidders commit to pay their amount regardless).
RABBI CHAIM HILLEL RASKIN ROV OF ANASH - PETACH TIKVA
