Sneakers: Footwear with style codes — StockX, GOAT, and eBay. Providers: stockx, goat, ebay.
Trading cards: Sports, TCG, and other cards — eBay comps first. Providers: ebay.
Vinyl: Records and pressings — eBay until Discogs is added. Providers: ebay.
LEGO: Sets and sealed boxes — eBay resale signals by set number. Providers: ebay.
Custom: Anything else — free-form notes and eBay search. Providers: ebay.