1. I don't think the guide is needed, the process is simple. However, maybe instead of keeping the text "Go to app", you should change it to "Connect to MetaMask" because it's not clear what you should expect after clicking that button in the first page. I deleted MetaMask, and went to the app.Ethconomy.io page, and no errors were shown, new users wouldn't know what's wrong and why they are not able to make an order.
2. If it's a P2P marketplace, it should mean there will be sellers as well... so yes the current rates are attractive, but I'm pretty sure with more sellers, there will be better offers.
3. Consider adding BTC. If you have no plans to implement wallets for users etc. Add the ability to spend from your Trezor/Ledger nano directly since I'm not aware of any bitcoin wallets similar to MetaMask.
4. I would like to see ASOS UK (GBP cards). Currently, the only service offering these is Bitrefill and it's not discounted. You should also consider taking off Airbnb gift cards or add a warning at least, these cards are known to be only redeemable in the U.S.
5. It definitely looks like you've put effort into making the site, so it does look trustworthy. Positive reviews from users who used the site would increase the site.