I bought a car for my daughter and within 2 months sold it. The smog was still valid (90 days valid certificate) from me buying the car from the previous owner. As long as the smog certificate is still within the 90 days, can I use it for selling it to the next owner? Or do I have to re smog?

If the law says valid for 90 days then it is 90 days. And it appears it is good for 90 days from when you had it done.

(quote)
When you transfer a vehicle that is four or less model years old a smog certification is not required. (Determine the oldest-qualifying year model by subtracting three from the current year.) The four or less model years old rule does not apply to diesel powered vehicles. A smog transfer fee will be collected from the new owner . When a vehicle is more than four model years old, a seller must provide evidence of a current smog certification except when one of the following occurs:
• The transfer occurs between a spouse, domestic partner, sibling, child, parent, grandparent, or grandchild.

• A biennial smog certification was submitted to DMV within 90 days prior to the vehicle transfer date (a vehicle inspection report may be required for proof of certification).

Smog certifications are good for 90 days from the date of issuance.

Ref:
Smog Information (http://www.dmv.ca.gov/vr/smogfaq.htm)

The smog certificate expired 14 days after the sale. They did not transfer the vehicle into their name until 6 weeks later, when it had a blown head gasket and wouldn't pass smog. The believe I am responsible since I am responsible for smog.

Did they know or were they aware of the expiration date ? Its hard to imagine they would wait that long and still expect it to be good. From your end when selling the car the smog was still valid so it appears you did satisfy the law.

The smog certificate expired 14 days after the sale. They did not transfer the vehicle into their name until 6 weeks later, when it had a blown head gasket and wouldn't pass smog. The believe I am responsible since I am responsible for smog.

Direct from the website:

The number of days you have to transfer ownership of a vehicle you purchased or sold.

Buyer
If you purchase or acquire a vehicle from a private party, you have 10 days from the date of sale to report to DMV the change of ownership. Transfer fees must be paid to DMV within 30 days of the purchase date, even if you do not have all of the required documents. Failure to pay your fees to DMV within 30 days will result in transfer and use tax penalties.

Seller
If you sell or transfer ownership of a vehicle to another party, you have 5 days to report the transfer to DMV. You do this by completing a Notice of Transfer and Release of Liability.


Ref:

http://www.dmv.ca.gov/vr/vr_info.htm#BM2523

DMV in my city states I did all the right things (release of liability, bill of sale etc), DMV in their city says I am in the wrong. 2 DMV's 40 minutes apart, different answers. This guy knew I had just bought the car for my daughter 2 months prior and was selling it when she wanted to move and hadn't paid me for it yet. So, technically he knew I had just went through the whole process of smog and paperwork and was just passing everything to him that I had just finished myself. I appreciate your input. Thanks. :)

Tell them no you are within the law ( unless you want to give the money back on the car)
1. where they aware of the gasket issue on the car when they bought it.
2. you meet the requirement, they did not register it in time, that is not your issue,

There were no known head gasket issues when I sold it. As a matter of fact, he told me that the car has ran great and has gotten excellent gas mileage for his wife driving quite a distance every day for her commute, until now. I told him I was not comfortable paying for repairs when they have used it for 6 weeks.