Buy it ... $5500 sounds about right for 75 uniques per day for a teen / mainstream type domain.
You might be able to get them down to $5000 or even $4500 ... but don't push too hard ... they may change their mind and raise the price.
Another method to try, which may yield even steeper discount, is put the purchase on the backburner for say a few months and then try again - sometimes people have had luck doing that ... but the risk is the domain will be sold and/or repriced higher by then.
No simple answers ... BuyDomains is in no rush to sell most of what they have ... and honestly, $5500 is reasonable consider the type-in traffic the domain supposedly gets; bet it has a nice OVT score too.
In the meantime, check out my selection of quality domains - many repriced.
http://www.domagon.com/forsale/
Ron