If you're set on farming and are accumulating mats faster than they sell when listed at market price then it's perfectly rational to want to move more volume at a lower price.

Say you can sell 20% of your weekly stock at market rate. What do you do with the remaining 80%?

1. Reduce the rate at which you farm so you can start selling off your backlog
2. Sell the remaining 80% at 50% market price to someone willing to buy in bulk

If you've invested in multiboxing then you want your accounts to be at 100% utilization when gold-making. There aren't many activities besides farming that afford you that opportunity, so (1) isn't going to be very appealing to you.

So, (2) it is!