tl;dr - scambots and scammers are always meanies. People are not.
There are a lot of scam artistes and low-ball goofballs when you are selling via CL.
Ignore them, and be basically edumacated so that you know that check/courier/whatever is a total scam and you ignore it. They can't send their scammy check and movers if you don't tell them where to send it (or you could give those ones the address for the local police station - do NOT cash the checks, then you are involved in the fraud. If for some reason you get one, take it to the police.)
It's basically noise, and when selling, you have to learn to filter it out. It's hardly limited to CL.
State in the ad (you may have) that price is firm, you already stated cash only, neutral location, etc. and simply ignore any reply that ignores those parameters. The ones that don't ignore it are the real people, who are out there. But the scam artists are out there all the time, replying to every ad, so there will be a lot of those to ignore for every real live buyer response. And real live buyers may take time, because (not being bots) they have to see your ad and see that you are selling what they want to buy and that they can reasonably get to a location near you without it being a $100 trip for a $20 item.
Don't bother to write back and point out that (thing you said in the ad already) is so - just spam-can it and move on. If you did not state that the price is firm, and you want to reply to the low-baller, you can, or you can just wait for someone who isn't a low-baller to come along. The congenital low-baller will offer half the price even if you make it very clear that it's non-negotiable, and the only thing to do with those is ignore them.