what is the theory behind the custom of no haircuts during the omer but it is ok to buy new items?
Shalom, Thank you for your question. The laws of semi-mourning during the Omer period have evolved over time, differing in different communities. Originally, as recorded in the Shulchan Aruch (Code of Jewish Law), only marriages and haircuts were forbidden – they are both major parts of the laws of regular mourning for family members. Other things that bring joy were added in some places – with dancing and festivities being forbidden in many places (see the Magen Avraham). When it comes to buying new items – this for of semi-mourning is found mentioned in the days leading up to the fast of the 0th of Av. It limits the happiness of the actual purchase, as well as the blessing said over new purchases, Shechianu. It may very well be that some communities took that practice, and applied it to the (lesser) mourning days of the Omer. However, many places follow the practice to allow buying new items during the Omer. I hope this is of some help. Blessings.