Fair Price of Drive Side Chainset?

split the rings and crank so you have three items, you'll maximise the money you get.

Drive side cranks are like rocking horse shit compared to left hand ones. I'd suspect you'll get a decent enough price for it, pop it on ebay on a 30day auction. You should be however kicking yourself for buying the whole chainset as left hand side shimano cranks are two a penny. 

Did you not look for a left hand crank before buying chainset. I suspect you would have much better chance of picking on of those up for 20% of the chainset price, than selling the remainder for 80%.

Most likely sellers are people buying stages power meters.

What would a fair price for 3 second hand car tyres be? 80%?

I would eBay it starting at 99p with no reserve.  The market for current bike bits is liquid enough that you'll get the best price.  I'd be surprised if someone pays 80% of the new price though - your buyers will either be:

- Replacing a worn big ring, i.e. looking to save money on a part that costs £100.
- Putting together a budget chainset with an orphaned NDS crank arm and therefore looking to spend less overall than the cost of a new chainset.
- Looking at fitting a Stages power meter to a new bike build.

Unfortunately, most of your audience will be the first of those three, and a quick eBay search will reveal that a complete open box chainset only makes about £100: http://www.ebay.co.uk/itm/282417784602

I think you'll be lucky to get £80 for it.