I've heard of these two terms, and I know they're both somewhat correlated with scamming.
What are differences between these two?
From the best of my understanding:
When you scam somebody, you find a new, guillible TF2 player who has earbuds (or an unusual, or another very valuable item), lie to them about their value (tell them that they are worthless) and offer very little for it. Offering a Paypal payment and then failing to go through with it (and viceversa) also counts. Here's SteamRep.com's definition:
A scammer is anyone that uses deception, thievery, lies, or anything that is underhanded to take advantage of or steal from another trader. See below for more comprehensive list of what qualifies for SCAMMER status: (snip; non-comprehensive list folows)
- Charge-back or Reversal (buy an item from the store, trade it away, chargeback purchase)
- Quick-switching items (offer to trade one item, trade a visually identical/similar but much less valuable item instead)
- Receiving the item and not paying (offer to trade for cash, get item, don't pay/pay and get refund from PayPal/etc.)
- Spycrabbing and then not paying (gamble an item away, lose and keep the item)
When you shark somebody, you find a new, guillible TF2 player who has earbuds (or an unusual, or another very valuable item) and tell them that you'd like to have it; you'll even give some weapons for it. The sharker banks on the new player being wanting for that annoying bow and arrow thing enough to part with a bunch of pixels in exchange for it. The player doesn't know better and agrees to the exchange, but the sharker didn't actually lie so it is not scamming. The trading "scene" places the blame then on the gullible player for being, well, gullible and agreeing to a bad trade. Obviously he should've known about steamrep.com and backpack.tf and tf2outpost.com and why the hell buds are worth that much money anyway.
When you lowball somebody, you try to trade an item for moderately less than what it's worth. Typically used by sellers when they don't feel like accepting somebody's offer. This is also not scamming.
Lowballing is offering less on an item Greed + Selfishness are the characteristics of someone like this.
Sharking is taking advantage of someone and offering less on an item Greed + Selfishness + Phycopathic are the characteristics of someone like this.
Pro tip : Phycopathic means you don't care about other peoples well being not being crazy.