Like increases to your stats, increases to your resists from items versus buffs are also capped separately and stack. The cap on each of your item-based resists is 1 + (your level/2). The cap on your spell-based resists is also 1 + (your level/2).

At level 50, your item-based and spell-based resists are therefore both capped at 26%.

Your race-based resists and armor-based resists and realm ability adjustments are added AFTER your item resists and buff resists are capped.

Therefore a level 50 leather-wearing norseman with avoidance of magic II, 26% cold resist from items and 16% cold resist from a buff has an effective cold resist of 6+26+16+10+5=63%, although only 6+26+16+5=53% will show up in the bonuses window.