I have verbally given acceptance to an offer which I think is a very low for me.Since this is my first job change and I like the current position in the company ,I said yes.The HR reverted back with background check form and said that after the check the offer will be given in writing. Is it too late to say I want a better offer.Should I fill the background verification form and wait and say I want to reconsider when they give the final offer in writing? . I don't want to blow them off,but I think I could do better
You are quite late in the game for salary negotiation in my opinion. If you accepted their offer, even verbally, they set aside certain amount of funds to sustain your existence in the company. Now, on the 11th hour, you come back with a counter offer, you had better have a very good leverage that makes them not being able to live without you. Since you mentioned first job change, I am assuming you are in the junior stage of your career. So, this company not being able live without you is not a likely scenario. So, tread the waters carefully at this stage.
Having said all this, you can go back to them, before handing them the background check forms and say something like:
After our last talk when we discussed my compensation, I did a little bit more investigation and found out that, this salary (or hourly rate if you are being hired on contract basis) is significantly lower than the industry average in this area. If we can not come to a mutually agreeable compensation, I will not be able to work for your organization
And leave the ball in their court. If they come back with a better offer, or ask you how much you expect, you can tell them what you want, in a tactful way, by supporting it with data obtained from a reputable source (glassdoor.com for instance in US; since I don't know your location). But at this stage, be prepared to be dismissed from the hiring process and probably not being able to work for this company, as long as the HR and the hiring manager is at their same position.
Should I fill the background verification form and wait and say I want to reconsider when they give the final offer in writing?
I'd tell them before that, nothing is set in stone so long as it's just a verbal acceptance. Be prepared not to get an offer at all though because normally you would have already negotiated before accepting even verbally.
Judgement call on your part, how badly do you need the job versus how badly do you want more money. If you're only going to try and negotiate a small increase it might not be worth the risk. If you want a substantial increase then it might be, because not only will you start off with more pay, but future negotiations will be about more as well. People who start a job with low pay tend to stay under the average until they leave that company. Because quite often raises are done as percentages.