How to effectively submit an offer 15-20% less than asking price

We are buyers not working with a buyer's broker and want to submit an offer on an apt that we believe should be about 15% less than the current asking price. There are some issues that are price drivers downward such as being on the first floor and limited light in one of the rooms, etc. My question is whether we frame our offer in context of those issues and use other apts on the market as... [more]

We are buyers not working with a buyer's broker and want to submit an offer on an apt that we believe should be about 15% less than the current asking price. There are some issues that are price drivers downward such as being on the first floor and limited light in one of the rooms, etc. My question is whether we frame our offer in context of those issues and use other apts on the market as comparison. The actual apt was sold several years ago, but price isn't available that I've seen. Also, how do we attempt to avoid coming to a verbal/email agreement on price, but then having a claimed higher offer emerge while we're having the lawyers work on a contract? Assuming we go up to our absolute max during negotiations, we won't be in a position to then raise it again...or want to with those tactics. Appreciate any feedback. [less]