I'm sure many more experienced sellers can offer you advice (and hope they do so)
Firstly, I'd recommend you buy either or both of the JM and Tim Brown Price Guides - they provide an indication.
Also, start pricing items with reference to last sold prices on here, on Evil Bay or Popsike etc
It takes time to do all this, but you pay lower sales fees and you can sell at your own speed and requirement.
As you say, if you ask someone to so it (a main dealer like JM / Pat Brady etc], they take a fee, BUT they may well reach a wider audience and achieve higher prices ie the price difference achieved maybe more than the fee...........
Please also remember to consider
Packaging
Postage
Insurance Cover
which countries you may or may not sell to
and
importantly how you wish to be paid
Hope that helps