IIIA should go for between 300-500$ depending on condition, whether it comes with trauma plates, wear, and date of manufacturing. Kevlar vests and most body army has a shelf life just like food and usually the company who creates the vest will tell you how long - the safe bet is usually around 2 years to rotate and replace a vest in LE/Mil use.
Now does that mean a 10 year old vest wont stop anything? No. However it might show a loss in performance of what it is designed to do. The exact level of deterioration would be unknown after the shelf life has expired so when you put on an "old" vest you could be wearing IIIA kelvar... or just another piece of heavy cloth.
I haven't seen pictures of the vest but I think a fair price would be anywhere from 250-300$ if its in good condition - based on the level of protection and age.