when it comes to selling a used set...please give the buyer some idea of level of completeness. At a minimum, are all the Minifigures included and an overall assessment of brick conditions: played with, adult owned, display only, etc.
Get an idea of value by hunting for the set # and then use bricklink or ebay for valuation
The best way is to build the sets and provide pictures.
If you don't think it's worth the trouble then loose bricks in good condition is like $7+/lb
If you're looking to get the best pricing...then you will need to put in the work to get the above details. Few will pay even close to RRP for a used retired set that is full of uncertainties.
A shortcut to the above is to line up all the manuals and/or make a list, take a picture and post it here and see if anyone makes an offer (which will likely be about 50% of the market price at best)