If the sellers ad did not state "electric payment only" or "electric payment preferred", then your "I'll take it" trumps all other offers. The way it's been for decades, and should continue, unless the seller states otherwise in his ad.
Seller is not trustworthy, and I would not feel bad about giving him bad feedback.
You guys that say get with the times, electronic is the only way...do you remember what a handshake means? Have you ever heard the saying, "a man is only as good as his word"?
I prefer USPS Money Order, or Cashier's Check when I sell on classifieds. If I get screwed using one of those 2, then...my bad. I also state in my ads, that first "I'll take it" responding to the ad, gets first crack at it.