Are these stats accurate?
Visitors to this Sedo Listing*: 134
Unique Visitors per month*: 7,846
* Data from the previous 32 days
Previous Offers for this Domain: 13
If so, the traffic alone makes it far more valuable than $300. But beyond that, the domain is good enough itself to justify a better price. I would consider a counteroffer of $2000-2500 and settling for $1k range. If traffic is good like it shows, then you may even double these numbers or more.
8k visitors a month, may earn you $80/mo for instance. Considering that names are selling for between 3-6 years revenue, that name is worth a minimum of $2880.
On a related note regarding income, I think the PPC ads its showing for you are poorly targeted. Motorway Maps is a term that sounds very foreign to Americans, yet the most visible ads on your parking page are for US state maps.