I am wondering if PTAs are allowed to sell advertisement for businesses to raise money?
A PTA can sell advertising but it would fall under the UBIT (unrelated business tax), which would require extra filings.
Is the product or service being offered one that is aligned with PTA's purposes? For example, promoting a healthy food product or activity for kids clearly would be, while an oil change, probably not so much; a discount on dangerous or harmful products is something that your PTA would not want to be associated. These are business judgments that your PTA's board of directors would have to make weighing the benefit to the PTA and the membership as well as the needs and circumstances served by your PTA.
Second, there may be other businesses that might welcome a similar opportunity. While you don't have to try to contact every business in town, you may want to put the word out generally before accepting the first opportunity that presents itself. Finally you may want to include a statement to the effect that the offer is being made available to your members as a convenience to them, but that if they choose to take advantage of it the business relationship is strictly between them and the business.