To try and answer better for the OP
The best I can say is each transaction on CL is completely different, as it should be treated.
100 great experiences does not guarantee that experience 101 or 102 will not be a horrible one where someone is attempting to scam you or worse.
Keep your guard up to a certain degree, not do much that the other person is put off by it. But do let it be known your no fool, and will not be playing anyone's sucker for the day.
Do your homework- know what it is your going to look at inside and out, if it's new to you do a good Internet search before heading over.
This way let's say your buying a certain type of bass that is well known among its owners to have very sticky & scratchy pots.
Knowing this before arriving allows you to ask if the seller has already fixed it, if they're functioning, or whatever but at least now you know that extra attention should be spent looking the controls over. Whereas before you may have just turned them up once and missed it. Just an example.
Try and get a fair idea of current value from different sources, check retail and the used market and asses the bass based on that and it's current condition.
AVOID MEETING AT NIGHT.
Unless its indoors or very very well lit.
I learned this the hard way, I met in a gas station parking lot and it was fairly well lit.
I looked this bass over very well even pulled the pick guard off to check for any crazy routing or strange wiring, everything looked awesome, the bass played great we hooked into the plug outside of the building and played from the amp he had in his van.
This thing looked brand new in that parking lot and I drove home thrilled because I got a great price on it.
However when I got home under normal lighting it was one little detail after another that was so obvious I couldn't believe I missed them.
I'm still happy because it's one of my favorite basses and I got it for cheap. But my point is shiny paint and plastics have a way of playing tricks on you in artificial light.
Day time is best or inside of a store.
If they insist on night just insist on a good place to look it over. Or maybe bring a good flash light.
I keep my purchase cash separated from all other cash and in s separate pocket, you don't want to pull out your entire pay check and start thumbing through the hundreds to find your way to the 20's. & 10's for example. Count it before you arrive, if you negotiate further just deal with that as it happens and pocket the extra, sometimes I'll walk away to situate my cash.
Follow your gut, if you feel nervous and you can't shake it, just cancel. It's not worth the risk.
If you aren't sure maybe try turning the tables and reschedule to a different area on your terms.
Most sellers are going to let you know upfront real quick of they're willing to haggle over price.
The ad is your first hint. If it says OBO he should expect negotiations.
Most people will tell you they're firm though on the phone or Email right up front. If that's the case I wouldn't push too hard if you really want the bass. If its not gonna kill you to possibly loose the bass then by all means play your hand and see where the cards fall.
Don't allow anyone to rush you, your making a purchase for something you may potentially have the next 25 years possibly and your about to drop a few hundred dollars on it if not more.
Take your time (within reason)
If they're rushing you and you feel pressured walk away, or ask to come back when they have more time. You may loose the bass doing this but with most, there will always be another.
I like to have options, if I'm looking for an MIM I may set up a few meetings. And I'm upfront with each of them that I'm looking at others and making a decision at another time.
They'll usually tell you it may be gone, just let them know that you appreciate their time but don't expect anyone to hold anything for you.
If its gone it's gone. If not great.
I wouldn't do this unless they know upfront and are not going out of their way to meet you.
Best way to treat these situations is to just treat everyone exactly how you yourself would like to be treated.
Keep in mind CL is FULL of window shoppers and teenagers with no money and they LOVE to call and talk about basses, the guy your meeting has probably dealt with his fair share of BS and low ballers before you called.
It gets frustrating and try and understand that they have no way of knowing your any different than the last 16 year old kid waiting for his allowance to buy something and told 30 people he's buying their bass but flaked on them.
Ok that was long,,,,Sorry but I felt I should answer the original question better than just saying to go armed.