The TBT Marketplace doesn't work like the shop. You can get an apple for 199 TBT, but since they are so rare (opposed to the other fruits), 199 TBT (or 200 TBT) is not the ideal price. It works like the supply and demand system in real life. When the supply is low, but the demand is high, the prices should be higher. Although the pricing guide is unofficial, most users follow the pricing guide. So that's why apples are mostly 800 to 1,000 TBT. Back when they came out, they were like 5,000 TBT, but that's because the demand was higher as the supply was lower (and there were more TBT in circulation). But since the demand for apples is ridiculously low now while the TBT supply has dropped, the ideal prices have dropped (which was why I was able to get 11 more apples for selling glow wands). But people are still not willingly to sell at the shop price.
As for the infractions, you always lose 20 TBT Bells for each infraction you receive. It's not much if you have like 5,000 TBT, but it's a lot if you have less than 200 TBT.
I did see that post before it was taken down. Giving out suggestions on how much people would pay for a collectible is one thing, but crashing peoples' threads, telling them not to sell them at a certain price or telling them to boycott the thread is disrespectful. Imagine if you were selling a chocolate cake for 1,000 TBT, then someone says "everybody, do not buy from this member unless if he/she is willingly to sell for 130 TBT" or "please do not price it higher than 130 TBT". That would hurt your feelings or annoy you. A proper way to tell people to lower the price is "the pricing guide has a median price lower than 1,000 TBT" or "you might want to lower it to sell faster". In general, crashing threads is bad for forums, especially trading threads or shop threads.