I believe some people who were planning to upgrade from their current 6,1 to a 7,1 weren't expecting the base model Mac Pro to double in price. This has caused some to hold on to their 6,1s instead of offloading them cheap to make room for the 7,1. Since the price gap between the two machines is so large, it has caused a slight increase in the 6,1s pricing.
This is my theory anyway. It could be the resellers on Ebay just bumping up the price, causing a false sense of increased value.
Given the 6,1's specs, its not really a bad machine. It's comparable to the 2018 Mac Mini in terms of performance... except it has a higher RAM ceiling (128gb vs 64gb), upgradeable internal storage, and upgradeable CPU. The 6,1's dGPU (even in D300 form) is better than the Mini's integrated graphics. And the final plus, is it's cooling system is much better than the new Mini's. The only limiting factor is TB2 vs TB3.
One final note... it's hard to say when Apple will drop support for the 6,1. It's been around since late 2013. I think the 2012 models will be dropped from 10.16 this year. Who can say when the 2013 Apple computers will fall to the chopping block.