Check where the domain gets its traffic and why there is a decline in traffic in the last two months.
If domain has lost its main source of traffic, there is a chance that in a couple of months it won't be worth a cent.
Also, don't forget that the domain is a .com.
You should check whether the domain has a value to someone for reasons other than parking. There is a chance that the previous owner might want it back (and, yes, some people might take 1-2 years to realise they want their domain back again)
In short,
before you decide to sell, try to understand why someone might want your domain and price it accordingly. I bet the domain has a meaningful name value and that's why you got a $500 offer straight away.