There is nothing you can do about sun bleaching - the best tip here is not to buy one that has been sun damaged in the first place. You will need to get more savvy about buying in the first place. There are so many Dysons available on Ebay, Gumtree etc, but choose carefully before you commit to buying. Only buy if there are many good pictures showing all angles of view so you can see any damage before buying. Ignore crappy adverts with one poor picture, they don't deserve a sale and you don't know what crap you are buying.
Never pay more than £20 for a Dyson Dc07 which is spares or repairs and £10 for a DC04 spares or repairs. You will never get you r money back when you come to sell, as a new aftermarket motor for either of these is £20 or more off Ebay. There is a top price that you can ask for a refurbed Dyson, and its usually about £80 for a DC14, £60 for a DC07 and £45 for a DC04 and they need to be IMMACULATE to get these prices.
If you have to replace motors and clutches, brushrolls, hoses etc you will be running at a loss, so you MUST choose carefully before buying, and not keep on changing your mind. Don't retract your bids either - only bid when you are 100% sure you want that cleaner.
How would you feel if a bidder bid on your vac and then pulled out at the last minute, ruining your auction and costing you the listing fees and an unsold Dyson, or one that you then have to sell at the lower bid price after the high bidder retracts their bids.
I used to spray a bit of back to black on the interior of the bin, but it is only a temporary fix to sell the vac with, and doesn't last long when the cleaner is then used.
Also, don't fit the wrong motor for the vac, like you have - it could malfunction for someone in use and cause injury or damage. The motor retainer is there for a reason, and should not be omitted. You should only sell a vac that is SAFE for people to use and definitely not "bodge" it up with ill fitting parts - I never did this and you could land yourself in trouble if someone gets hurt by the motor if its not encased properly.
The clutchless DC04's used different motors and housings to the clutched DC04's as you have found out. I didn't realise you had bodged it with the wrong motor. My advice is NEVER to bodge a repair that you are going to sell to someone else!