Everyone has a different definition of what a "lowballer" is. Some people think a lowballer is anyone who doesn't agree to pay the asking price of the seller, which is laughable. It's bullsh/t anyway...if you don't like what someone offers you, tell them no and what you'll take. Pretty simple actually...leave the emotion out of it. You're probably one of those guys who puts "make an offer" on some of the stuff you sell, then get all pissed off when the offer isn't what you want. Those are my favorite.
It's all about negotiation. If your feelings get hurt from that, then grow the f/k up. That's the issue with people who ***** about lowballers. It's an offer, nothing more, nothing less. The only thing I can sympathize with in your post was that you did agree to a price on the phone and he asked if you'd take less when he got to your place, which is b/s. I would've stuck to the original price, and required an extra $10 to help him load it...if I could afford to wait for someone else to come buy it. If not, I would've told him I had to get the $40 and if he wanted it, that's the price. That's what I always tell someone who offers me less than the minimum price I need to get for something...I have to get $XXXX for it. I usually put my selling price a little above my minimum price...so I can drop a little but not too much.
Bottome line is, he was all set to pay you what you wanted for it and you told him no, you lost out on the $40 and an additional $5 b/c you're immature, unstable, and must be a real ***** to deal with. You have to have a real hair trigger temper to act the way you did...like you were just looking for a confrontation. Sounds like a losing proposition all around. But hey, that's just my $0.02