Unless the listing says a condition, the assumption is mint. So if the card is amnything less than mint, that should be communicated. I have done several deals where I have let my trading partner (or buyer) know exactly what I'm sending. If it's a purchase, anything that is not mint is a "throw in" and the buyer can do with titwhat they want - it didn't cost them anything. If it's more of a high -ticket item, I price accordingly.
On the flip side, I have received some 90s cards that were not in the condition I expected. I contacted the traders about this and we worked out any issues. But yes, it's disconcerting to receive a card that is expected to be mint, only to show up with crease or a water/food stain on it.
Again, assumption is mint (or really close to it for the older cards) unless condition is listed in the FS/FT listing. It's incumbent on the sender of the card to disclose any condition issues prior to completing the deal.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).