Amazon usually takes one of two positions when it is selling a particular product. You can tell what Amazon is doing by looking at Keepa 

1. They Control The Buy Box & Don't Share (Don't Try To Sell These Products!) 

If they have an exclusive (usually smaller brands) then that is not a good time to go on that listing. Amazon will not let anyone else win the buy box (where p80%+ of Amazon sales happen) and that means you will get very few sales. 

Keepa Will Look Like This on Listings Where Amazon is Not Sharing: 

You can see that Amazon is always adjusting its price to make sure it has the lowest price (and thus wins the buy box). If you hover over the Pink Diamonds (buy box wins), they will all say "Buy Box by Amazon". 
You can see that Amazon is always adjusting its price to make sure it has the lowest price (and thus wins the buy box). If you hover over the Pink Diamonds (buy box wins), they will all say "Buy Box by Amazon". 

2. They Let Other Sellers Win The Buy Box With A Lower Price (Sell These Products!)

If Amazon has let an FBA seller selling new, undercut the price and if they do not match, then it is a good time to go on that listing.

Keepa Will Look Like This on Listings Where Amazon is Sharing: 

You can see that Amazon keeping its price steady and not matching the lower prices. This means 3rd Party FBA sellers like us can undercut Amazon's price and win the Buy Box. If you hover over the Pink Diamonds (buy box wins), they will all say "Buy Box by 3rd Party FBA". 
You can see that Amazon keeping its price steady and not matching the lower prices. This means 3rd Party FBA sellers like us can undercut Amazon's price and win the Buy Box. If you hover over the Pink Diamonds (buy box wins), they will all say "Buy Box by 3rd Party FBA". 

Please check out the course section on Product Research In-Depth (Competition lesson) for more details on this :)