It always happens, you always get lowball offers even if you say no lowball offers. I just ignore it and wait for a better offer or DDW it and put it in my shop then there is no offering happening.
Just ignore their offers. No points wasting your time neomailing them or even rejecting their offers, really.
It's funny, on my shell account I put up some UB I got from the igloo and up on the trading post. I usually put something noob in the description then I tend to get a lot of offers. When they offer something crappy I usually neomail them telling them if they can get me all the jellos or something + 10k NP then I will sell it to them. They usually fall for it and end up getting all those jelly for nothing, wasting their time, mine too but I got amused. :]
lol, I usually block them. after that I send them a neomail saying:"Hi and welcome to my blocklist =). I hope you like it here^^"
I ignore them & reject the offer. I know some people only offer on expensive lots so they can get the soup faerie avatar. And for the other who are trying to /scam/ or resell, well they're just wasting their time.
I will accept anywhere from 5 to 10k below TP. Like someone, I think Hally, said earlier, I am ok with them profiting from it; I igloo so I'm making a pretty decent profit since I only sell ubs on the trading post anyways. However, if it is ridiculously low, I just reject them and block.