TCGPlayer
TCGPlayer is often useful for US-market singles and playable card liquidity. It can differ from sold-comp or European prices when supply is regional.
Cardmarket
Cardmarket is a strong signal for European market pricing. Currency conversion and shipping can make the same card look higher or lower than a US price.
eBay
eBay is useful for broad availability and unusual items, especially graded slabs and sealed products. Listing prices are not the same as completed sale prices, so confidence matters.