We usually aim to pay 70% of the expected re-sale value.
The 30% covers our costs of shipping and insurance of used gear in for evaluation (and back again if a customer turns down our estimate); cleaning and servicing of gear prior to re-sale; advertising; warehousing; the cost of providing for most items a 6-month warranty - which includes a 30-day returns period; staff wages..........and the boss expects us to turn a profit!
I guess as with any service you outsource, plumbing, carpentry, car mechanics etc - the bottom line is: if your time is worth more and you want a quick job you get someone else to do it!