I think if someone was willing to pay regular price for the game, they wouldn't wait until they saw your offer to save a few pennies. People who are willing to pay full price already have it, and people who are waiting for a deal will probably wait for the Steam Sale and get it for likely half what you're asking if not less. Good luck with it, it's certainly a good game but people don't exactly come here looking to save pennies.
The thing is, it's not regular price. It's a good ~75 cents cheaper, which is a big deal for anyone with an incredibly limited cash stack, speaking from personal experience. There's a reason the current market price for Undertale is 4 keys, you'll rarely find it cheaper. Your point about the Summer Sale is definitely valid, but if someone doesn't wish to wait, this option is open to them. I don't see the harm in creating a thread in which I specifically state an item and my asking price. If someone wants it, great, but if not, they can just move on.
There's no real harm in it, and I wouldn't have commented at all except you basically mentioned the difference between your copy and the "Steam" copies is just the market tax. The difference depends on the keys, but with 4 "normal" CS:GO keys it's maybe a 5% discount, with the "additional costs" of having it as a 2nd-hand gift (which has a few downsides like no refunds and not actually being purchased by your account). It's worth mentioning I clicked the topic to see if there was anything else you wanted, because Undertale IS a game I want. If you have the patience to wait you might find someone who would want this deal, it's just a really slim market because you'd need someone who is willing to pay like 95% of the cost now for a 2nd-hand copy but not regular price for a new copy by this point, which means they missed the previous bigger sales and can't wait a few weeks for the Summer Sale.
Good luck to you, here's a bump hoping you and someone else happen to line up and trade.
I do not accept TF2 keys or other games. Send me a trade offer for a fast transaction: https://steamcommunity.com/tradeoffer/new/?partner=83172395&token=_hY0pHKd.