There was an error accepting this trade offer. Please try again later. (28)

either steam needs some time (server busy or something), or the trade has already been accepted and the trade window keeps throwing errors at you anyway. ^^

EDIT: i just read error 28 might happen due to inventory issues. maybe your tf2 inventory is full or something?

EDIT2:

There was an error accepting this trade offer. Please try again later. (28)
This means: Items no longer available or unable to check if the item is still available.

Possible causes:
The items became unavailable after you opened the trade offer window.
Item server is dead and unable to check if the item still exists.

someone had this error earlier with a trade offer i sent.

Do you have many trade offers pending or declined recently?

I had this issue when trading with card bot. Sent many offers but bot was down, so my offers were stuck for few days and even after bot was up i kept getting error until offers cleared itself.

@killingarts: thank you for the info, might come in handy

@mullinx: I guess I'll just wait a little longer and see if it comes through tomorrow if others are having similar issues

@gonsi: thank you for your comment. I just have the one trade offer currently and none declined recently, so that doesn't seem to be the problem I'm encountering.