I would say its best for a seller to say how much they want upfront.
Unless they are offering the option of collection in person, or other payment methods such as bank transfer (not recommended) then they should inlcude all costs - either all inclusive or with each detailed.
e.g. xyz marker for sale £100 cash in person, £110 including postage & PayPal
Alternately xyz marker for sale £110 all inclusive or collected.
(That way you have the option of an extra £10 in your pocket if it turns out to be a cash sale in person or a bit or haggling space)
If the sale says xyz marker £100, plus postage you pay fees or payment owed, then its not entirely clear what the cost is to the buyer, and if they dont work out the fess properly the some sellers get upset when they get £99 and some pence.
On the other side of the fence people would get upset if they see something in a shop window for £100 but £120 comes up on the till because VAT was not included.
Avoid bank transfers and 'payment owed' as you lose all protection if something goes wrong.
Payment owed still has fees, however it is worked out at the source for the buyer.
For sellers I would say keep it simple and make it clear and easy for buyers.
All you do by showing a start price rather than inclusive or broken down is show an artificially cheaper price.
In the second hand market something is only worth what people are willing to pay, to the buyer the postage and fees are part of that value that they are willing or not willing to pay.
