This username purchase scam is beginning to get common and popular now these days; I can count about 3 good threads about this scammer just this year in this forum. What their intentions are is what I don't understand. In the last few threads, the scammer was trying to use a fragment, but this time around this scammer comes with a different approach, which looks like a setup.
I don't know Gemspace although I think it's not the first time I hear that name. Anyway, I would have done the same as you: distrust if something sounds too good to be true, and especially in Telegram.
Gemspace is an escrow website, which looks like a scam to me, and those who suggest using it are likely the ones in control of the website. It always requires both parties to connect their wallets to the website and grant the site permission. It all looks fishy. I have been offered a deal using different methods of scams before, but when the scammer suggested that escrow, it made me distrust him immediately.