If you are looking for something wider than 24mm for a good price, you cannot go wrong with the 17-40mm. If it's minty as you say and the price is right, I wouldn't hesitate to much. $200 saved is something, and a scratch on the bayonet mount is a non issue. As far as the rubber lip is around the bayonet is not cut (it could compromise the weather resistance), I would just take it.
If you doubt, maybe can you have a short test to check for any major optical flaw.
The 17-40mm is the only possible choice with Canon for a super wide-angle at a decent price. The only other options would be the 16-35mm, the 14mm, or the 17mm TS-E ($1500 to $2500).
I've bought dozens of second hand lenses since 20 years, and had only one bad surprise ( I bought a lemon 18mm 3.5 AIS Nikkor).
I would just try it, then buy it if the IQ is OK, if you feel you need something else later, just resell it, you won't lose much (if anything). That's the joy of buying second hand, you waste much less money.