If by "selling stuff online",you mean Craigslist,there's no good answers. 50% are phishing for email addys and phone #s,10% do it strictly for intertainment,10% are auctions looking for steals to turn a profit,10% intend to screw you out of it for nothing. It's not unusual for a few of remaining 20% to "horse trade". My usual responses are. Show me 3 others like it for sale cheaper and we will negotiate. Exactly why do you think I should sell it cheaper than I've listed it for? My favorite when they ask before laying eyes on it,"if I say yes to your offer,will you pay me when you get here and before seeing it"? For those that ask for "pitchers" or "more pitchers" of things like 4'x8'x1/2" drywall,20' of 3/8" #43 log chain with grab hooks or new set of .30 over pistons for 5.7L chevy. I don't even respond.
My late brother laughed and razzed me for 20 years over selling an old dirt bike. I bought it not running from a neighbor whose son had left it behind when he married. A little tinkering and it was running so I sat it out with a $50 for sale sign. Guy stopped,test drove it and offered $40. I refused and he left. Couple hours later he came back with $50 in hand. Meanwhile my brother had came over to sip some brews. When the guy walked up he said" the bike is over priced but I'll buy it." When he held out the money I told him "if you think $50 is over priced,you will consider $60 highway robbery but that's what I'm asking now." After turning red and lecturing me on the reasons I couldn't go up after telling him $50,I told him "the **** I can't,I just did.". The guy turned and went to his truck cursing with every step. After sitting in the truck a few minutes,he got out and headed my way shaking his head. Before he had a chance to speak,I said"the bike is now $75 take it or leave it and I'm not saying what I might want tomorrow". The guy silently stared for a few seconds,laughed and said "i'll take it" then handed over $75. My brother never allowed me to forget it and took great pleasure in telling friends about his greedy brother.