How do you estimate the value of an account? And how much value does it lose when having a 2day ban?

depends, most time the 2day ban will reduce the price by 30-40%

Ban in some cases could be 50%, as a lot of people buy an account to bot and with a ban already on the account, the rate increases.  Value is also determined by age, QP, stats, and networth.  Things like original ownership might increase value in some cases, but in most it just helps receive an interested buyer.  

Value of the account would depend on stats, quests completed, unique items like firecapes etc, and wealth(if any).  I've never sold an account so i wouldn't know too much about the 2 day ban pricing though:P

2 days ban will only decrease price if people are going to bot on it. If they want it to play legit on it, I think it will stay the same price, since after 1 year the ban will expire

2 day bans lose 0% value of the account if its "perfect" and requires 0 botting as its already completed, but if it still needs a lot of work done 2 day bans suck.