Hi,
Liquidity is quite a relative term, it is linked with marketability & proportions, & that too has multiple shades as whether you are more trying to contact the endusers or the re-sellers.
From my recent marketing experiments on domain promotions & sales, you have to pick the very best, not likable but hugely admirable & business oriented domain. With this if you have to make sure the domain you want to sell should reach the maximum number of people who are interested in that niche domain purchases or who may be.
Cutting it short, read some of the blogs at Namepros & Flippa regarding which niches are mostly bought & searched for. Those will certainly have liquidity, but again I assert that liquidity has quite a lot of things to do with promotions & reaching a great number of buyers who could be interested in your niche domains.
It is quite a Hard Work to do.
Best Regards
Daneyal F.