Do you think I could have a class action case against this bank?
An individual does not "have a class action." You haven't clearly articulated what you think the credit union is doing wrong, but you're free to try and get a consultation with a couple local class action firms to see if, after reviewing your written account agreement, they would be interested in pursuing a case. Keep in mind that the likelihood of significant recovery will have to exist for a firm to be interested in pursuing such a case.
Let's say I have a balance of $100 in my account. I spend $100 at Bestbuy using my debit card. I could see in my held funds $100 in a special section of my account and a balance of $100 shows on my account. That $100 on hold for BestBuy might take 3 days or more to disappear. Meanwhile if I need to overdraft and use my debit card to take $200 out of the ATM I have no problem paying the $35 fee. The thing my credit union does is once that $100 for Best buy clears (they already took the $100 ) you will be charged a $35 fee for Best buy and overdrafting if your account is negative.
So...I think your complaint is that the $100 charge at Best Buy should have reduced your balance to $0, so you shouldn't have been able to withdraw the $200. In fact, no matter what, you shouldn't have been able to withdraw the $200, but you apparently were and were charged a $35 overdraft fee. The Best Buy charge was then applied and you got hit with a second fee. The propriety of this practice will depend on the specific terms of your written account agreement.
No they did not advance me the $100 for bestbuy they took the money from my account because of the debit transaction and still charged $35 once it "went away" which is complete fraud!
It could be argued that it's fraud for you to manage your account in the way described. If what the bank did was proper under the account agreement, then you took advantage of those terms (which were known to you) by making a $100 purchase and then a $200 withdrawal. I don't know why the credit union allowed you to withdraw $200 when you only had $100 in the account, but that's neither here nor there. Having done this, you knew (because you knew the terms of your account agreement), it could be argued that you committed fraud by making the Best Buy purchase that you knew would be declined by the CU. Of course, I know you didn't strictly "know" what your account agreement says, but you're legally responsible for knowing.
As suggested above, you can take your written account agreement to an attorney to review.