Fit matters more than the label
«Ergonomic» doesn't tell you whether a chair will suit you. What matters is the seat height range, depth, backrest and armrest adjustment. If you can, compare the measurements with a chair you find comfortable.
Your height, desk height and how long you sit all affect the choice. Look up the manufacturer's maximum load for this exact model. Don't judge it by the look of the base or the chair's weight.
Check the mechanisms
On a used chair, the gas lift, tilt mechanism and locking matter. A chair that looks fine in photos can sink under load or refuse to lock the backrest. Look for a specific list of tested adjustments in the description.
Check the base for cracks, as well as the castors, armrests and any damage to fabric or mesh. Worn cushions can spoil comfort even if the mechanism works. Find out whether spare parts are available for this model.
Weigh the total outlay
Compare condition and contents, not just the brand. Add cleaning and replacement parts. For collection, check the dimensions and whether the chair can be safely dismantled without losing any fixings.