personally, low price and reliability – I don’t think next day delivery is necessary if you’re cheap, but within the stated period is ESSENTIAL.
I reckon parts trade-in would be a nightmare for a shop (how would you shift stuff on, and how would you value it ? If you sell it, will you warranty ?)
just categories – the big shops offer filtering by price or by discount size anyway, so a “sale” section is worthless IMO (super-cheap clearance stuff, maybe but the CRC sale section is way too big to read through). Note how many people moan about boasting big discounts on the index page, followed by only in XXXL or XXXS sizes getting the low price – these would fit well in a separate index like “clearance” IMO)