What would be a reasonable amount to offer below the sticker price of a Honda Civic sedan?

In other words, if the sticker price is $21,000, what is the LEAST the dealer would be likely to accept?

18, maybe 19

Depending on the Civic's trim level, accessories and options you can easily find its current market value here.

https://www.edmunds.com/honda/civic/2017/sedan/

Is it new?
Often there will be sales at 5 or 6 thousand off list or more. Watch a little 11 o'clock news for dealers in your area with sales on.

Honda dealers will not negotiate. If the sticker price is that, that is what you will pay

Expect to get around $1,000 off sticker on an EX. On an LX maybe $500. Go there on the last day of the month and you will get a better deal.

The dealer does not have to negotiate because there's a buyer right behind you so it can be "NEXT"!

It is hard to negotiate when they are such a hot seller… And dealers can't keep them on the lot… Always selling out.

They won't be stuck with excess cars on the lot… They go that fast.

$15,000 is a lot of money.