A fair price is whatever a willing buyer and seller can agree on.
I tried to look up original retail prices for that trike but I couldn't find one. I'm guessing it probably sold for around $1,200 but that's just a guess. My rule of thumb for used bikes is to start at 50% or retail price and go from there. That would be $600.00.
At 7 years old and with only 50 miles of use, I'm thinking this trike has negative utility to the buyer. He gets zero benefit from it and it takes up storage room. He'd actually be better off if you carted it away for nothing. How badly do you want it? If I wasn't worried about insulting the seller, I'd probably make around a $250.00 opening offer. I definitely wouldn't go over $500.00 all in price.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.