If Buyers CST no. not maintain in excise cum tax invoice and retail invoice both in sellers invoce and CST charged 2%. So there is any problem for sellers because of not maintain buyres CST no.?
During transportation, if there is some verification at check post etc, they may ask for VAT registration number (this happens only when it is specifically checked, chances are rare)
If there is no registration number, then there may be a problem while uploading VAT return (as some CTDs seek invoice-wise information)
If there is no registration number, then buyer might not be able to issue C form, so you will have to pay the difference of tax (if no C form is received)