Near the end of WoD I made a similar post suggesting what pets to invest in going into Legion, so here is what I believe will be solid pets to hold onto 6 months to a year into Battle for Azeroth. This is NOT a short term investment, so if you're looking for quick flips I'd suggest not buying any of these unless well below regional averages.
For the last couple weeks I've been buying these up to 70% of the region average to help build up a little inventory for myself as some of these are already rare during Legion. To stop from selling them too low too soon I've put them all into a separate TSM group prices at 400% region average, surprisingly I've actually sold a few of them at this high price.
- Nightmare Whelpling Source: 1% drop of Ysondre in the Emerald Nightmare raid. Drops in all difficulties.
- Fossorial Bile Larva Source: Very low droprate from skinning mobs on Argus. These are already a bit pricey and rare while Argus is current content.
- Rebellious Imp Source: 6% droprate from Mother Rosula in Argus. This rare is on a daily lockout and takes a few minutes of farming to summon each time. Worth a decent amount currently.
- Thistleleaf Adventurer Source: Rare drop from the Dreamweaver Provisions box from doing the Dreamweavers Emissary.
- Trigger Source: Crafted by Engineers. Costs 50 Bloods of Sargeras, you can often find these cheaper than the trade-in value of the BoS. The unlearned companion pet version is often cheaper.
- Wyrmy Tunkins Source: Reward from a Legion Archaeology quest chain. This quest chain is only available for a 2 week period every 26 weeks making this essentially a holiday pet that will be forgotten when Legion ends.
- Fel Lasher Source: Rare drop from Argus herbalism nodes. These herbs will have no notable use once Legion ends making it very rare for anyone to bother farming for this pet. Super cheap now so lots of room for profit.
- Eye of Inquisition Source: Less than a 1% drop from Felsoul Inquisitor in Suramar. Currently these mobs are killed for world quests that will not be completed as often after Legion.
- Sting Ray Pup Source: Conjurer Margoss the Fishing Vendor in Dalaran. Requires rep to buy and a fishing currency only obtainable from fishing in Dalaran. NOTE: The companion pet version requires reputation to learn, only buy the battle pet version.
- Orphaned Felbat Source: 5% drop from the Legionfall paragon chest for ever 10k rep after exalted. Currently fairly cheap.
- Ghost Shark Source: Sold from Ilyssia of the Waters the fishing vendor in Azsuna. Requires rep. Vendor and fishable currency are on a daily rotation so it is only possible to get this once or twice a week, takes a decent amount of time to get.
TSM shopping search string: p:2121; p:1723; p:2135; i:130167; p:1803; p:1886; i:132519; p:1887; p:2117; p:1719; p:1911; p:2050; p:2077
A TSM group import: p:2121,p:1723,p:2135,130167,p:1803,p:1886,132519,p:1887,p:2117,p:1719,p:1911,p:2050,p:2077