In my opinion:
I'd say $125 for the selling price if the bag is included, and add $50 if it has mulching blades and plug. I bought an HRC215K1 SXA (which is 10-15 years newer and is a much more desirable mower than an HR214SX) earlier this year for $200 and it came with all the items I mentioned, was in excellent cond. and the blades were brand new. You might have to discount the price more than the numbers I mentioned above because not too many people want to pay very much for a 30+ yr-old mower. I've been watching CL asking prices for the HR series mowers for a couple of years now, and have bought a few. Last year I bought an HR215SX Masters series (again more desirable than an HR214SX) and it also came with all the accessories mentioned above and only paid $125. Timing is also very important in pricing an item like a used lawnmower - earlier in the mowing season there might be a greater demand for used mowers, but this time of year most people have already purchased their mowers. Also, a large metropolitan area commands higher prices than rural areas. My price estimates are based on a large met. area. Good luck in selling yours.