I found the average price of a solo on amazon to be around 140. That being said, i dont know if its just me, but using an amazon price for comparison isnt really fair and is similar to giving an ebay price. While there alot of reputable dealers/distributors on both sites, there is also risk involved (some are older models, some are refurbished, having to worry about warranty service etc...). The average going price for a solo from distributors like vapeworld/gotvape is around 225$ but they do have pricematch, and it can be purchased for 160 from other legitimate sites like PIU and PV... I would say the average value of a solo would be around 140-150+20ish for your gong.
Although every version of the InH sells for 249, you can not use that value for older models even if they have never been used. I would say the value of a unused older model would immediatly decrease in value by at least 50 dollars when the new model came out. I think that a straight up trade of your solo/gong for an older InH used or unused would be pretty fair for both parties but i doubt youd ever be able to trade straight up for a new model inh. Sorry for derailing your classified ad, just thought id add my 2 cents.