This likely applies to all larger commission based purchases but do your large buys near the end of the month.

Why? Commission based jobs generally have bonuses and incentives for their staff to hit specific sales targets for the month. What this means is, near the end of the month you'll have sales staff trying a little harder to close deals and get in those last minute sales. What this means for you is a sales person more willing to negotiate a better deal on your purchase.

This is coming from personal experience as I worked in furniture sales for 6 years (two different stores) and tried it out a couple times myself.