He can farm pretty good with dark pact actually, but he shouldnt be farming in early-mid when he excels at ganking imo.
He can Gank + farm neutrals while he waits for the kill (farming incredibly better than most gankers, POTM, Pudge for example while netting lots of kills).
If you are doing great rush skady + str PT.
If you are doing OK, go for point boster and ogre axe and wait what happens, if geting raped by disables go bkb, if you are raping every time even harder get skady.
If you are doing REALY BAD get sange n yasha, cheap components paired up with slark farming potential will ensure you complete this item 100% sure, although the maim is unrelible and the other bonuses are just mediocre.
In all cases slarks need more hp pool so you need to buy "yes or yes" at least 1 item that grants at least hp (i recomend skady, sange+yasha, bkb).
Other items like vanguard (the regen + block aint neded mid game) or HoT( the regen gets totaly wasted) are not recomended.
Mom is just a crapy item on slark, he canot aford by any means the 30% plus in damage as he will die to one finger of death... Your invis does not makes you invulnerable to AOE damage and it does not last long enough.