A guy near me has a 14SB for sale for $150 that appears to be in good shape, so $300 may be a bit high. He said it was about 13 yrs. old.

The answer is:
What ever the market will bare.
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If the mower has the typical dirt that collects on a mower, someone might offer $30 - $40. If the mower is cleaned up and serviced to look like new, someone might offer $100 - $150.
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The level of marketing will determine the final price. Expect a low price if you park it on the corner of your yard with a sign. Expect a premium price if this is cleaned up and advertised on Craig's list. The shoppers who are looking for your exact piece of equipment will probably look at the Craig's list.