The groupset (shifters, derailleurs, brakes, crankset) is the single biggest price driver on a used road bike, usually a bigger factor than the frame itself. Here is what each tier actually means.
This is a dated snapshot, not a live feed. Prices on the boards moved after that; confirm on eBay.
Claris and Sora are entry tiers, 8 and 9-speed, found on lower-cost new bikes and older used ones. Tiagra (10-speed) is a step up in shift quality. 105 is the tier most riders upgrade into and where a lot of the decent used inventory sits: full carbon-composite levers, real racing-level shifting for a fraction of Ultegra cost. Ultegra is one step below Dura-Ace with most of the performance. Dura-Ace is Shimano's race-team spec, the lightest and most expensive, with a used-market price premium even years after release.
See what sits on each board in the current snapshot: 105,Ultegra,Dura-Ace.
Apex and Rival are SRAM's value and mid tiers. Force sits close to Shimano Ultegra in both price and performance. Red is SRAM's top tier, competing directly with Dura-Ace. SRAM's 1x (single chainring) setups also show up more often on gravel-leaning road bikes than Shimano's road lineup does.
Centaur and Chorus are Campagnolo's mid tiers, Record and Super Record the top. Campagnolo parts and compatible wheels (their freehub spline pattern is not the same as Shimano/SRAM HG) are thinner on the ground than Shimano or SRAM, worth checking parts availability before buying an older Campagnolo-equipped bike; seewheelset freehub compatibility for the full breakdown. See the snapshot board: Campagnolo road bikes.
A frame you like at two different prices is often the same frame with two different groupsets. Decide your groupset tier first, then filter frame material and size, not the other way around. It usually narrows the list faster than any other single filter.