Round here everyone uses leboncoin.fr for private sales (and lots of agents put houses on their first). I've never herad of pap.fr and have just finished a 9 month househunt.
I just looked on it and there are only 10 houses for sale in our town whereas on leboincoin there are over one thousand!
With the fees, it's generally accepted that buyers pay. The prices are listed agents' fees included and then you have to allow for the notaire's fees on top of that. We sold last year and when we agreed the market price with the agent, they concentrated on the net amount we would get then added their fees so it was listed at a really odd price, xxx thousand 437 or something like that. They did round up or down the centimes 
You shouldn't have to pay fees when you sell but if you lower the price so a bit that the over all cost is lower to attract buyers, the agents fees don't change that much so effectively it's like you are paying a bit of their fees.
I would recommend a notaire too. The house we just found was through a notaire and they were very proactive about sending out details which matched our requirements (although by letter, very old fashioned) and they sent out information regarding properties available with other notaires too.
You can have more than one agent (unless you've signed a 'mandat exclusif'). Some agents are really useless. When we were househunting we gave our details to about 14 different agencies and only 6 were proactive about contacting us. The others we had to keep checking their web sites and some even failed to turn up to appointments to visit!