You will get WAAAAAAAAAAAY more than 300. I just sold my Crosby because I wanted a Beval. A little back story on my Crosby: I was an original, before being sold to Millers that went under and now its Weatherbeater but the Crosby name is pretty much shot. So it had orginal french leather, made in Englad, clean and well cared for, yada yada. I took it in for trade and they offered me 300 bucks. The woman told me it would have sold on Ebay for 595.00. Which I had already tried unsuccessfully. So I sold it to a trainer friend of the store owner for 425 and put that money toward my Baval.
Anyway, if my trade in value on a 16.5 inch, plain flap, 12 year old Crosby was 300... your 2 year old, 17in, HDR with knee rolls will fetch much more than that, I promise you. So my advice would be, before trading it in, consider consignment. I couldn't do consignment because it was all I had. I had to sell it before I could get a new one. If you are in the same boat, use Ebay or Craigslist. I sold a crappy Regent on Craigslist super fast because it was cheap.
If you need any further help on how to sell, let me know. I have a personal interest in saddlery and have sold two saddles this summer! Its a real pain trying to unload tack you want, trust me! I know!
"There is something about the outside of a horse that is good for the inside of man." ~ Sir Winston Churchill