In my opinion the Blade 500 is pretty much worthless if you cant get spares. Due to the nature of the hobby you will need spare parts probably sooner rather than later.
Try checking the spares situation for yourself but as i understand it critical parts like the main gear simply are not available making the heli almost unusable.
For the 450 3D.. This (like the 500 3D) is quite an old model. Flybar helis dont have much value these days. Here in the UK a guy was selling brand new boxed Blade 450 3D kits compete with DX6i transmitter and charger exactly like the one your workmate has on eBay for £75 recently (that's about $100) :
Blade 450 3D | eBay. Prices in the USA are usually cheaper because you dont have the sales tax we have... so make your own conclusions.
Like I said in the last post, used RC helis is very much a buyers market.