Use both. Discogs is better when you know the exact pressing. eBay is better when you want the lowest price and you are willing to read the listing.
Discogs lists the release, the label, the year, and the condition in a set format. You can see what other people paid. That makes it the right place to confirm you are buying the record you think you are buying. Prices sit a little higher because sellers know the catalog.
eBay is a pile. The same album shows up with bad photos, wrong years, and shipping that changes the math. It also shows up cheaper, especially on common titles, because a lot of sellers are not pricing off Discogs. Read the description. If the photos are blurry and the grade is a guess, pass.
A simple split:
Digging in the Sales Crates runs one search across Discogs, eBay, CDandLP, and Turntable Lab. You still pick the seller. The page just puts the options in one place.