If a non resident must pay tax on the gross amount of a sale of a house in NJ then is it necessary to file NJ income tax to obtain any refund even though the seller lives in another state?

Sale of property located in New Jersey is subject to New Jersey income tax whether the seller is a resident of NJ or not.

Non-residents use Form NJ-1040NR to file their New Jersey tax returns.

http://www.state.nj.us/treasury/taxation/prntgit.shtml#git

If you have a NJ tax liability, you will probably be able to claim a credit on your home state's income tax. There is usually a special form or schedule to claim the credit.