buy bitcoin with usd instantly in exchange sites bitcoin ,each has different buying limits and conditions like Identity verification but it can be scam though ,localbitcoin it look good or why not open account in exchange coins like bitfinex or livecoin and deposit usd then buy bitcoin
Please do not go with any centralized exchange, once they become with the peak market circulation with them. If any legalization issue you find in your countries you will not able to being anoymous in the market.
Localbitcoins.com would be best choice for converting the bitcoin to cash and for buying as well as much knowledge.
Livecoin does not have problems with signing up since it is decentralized one.
That's right even me I don't trust in a centralized wallet maybe someday they will disappear together with your bitcoin storage.
If I'm not mistaken Blockchain.info wallet is decentralized bitcoin and ethereum wallet, right? because you are the one who holds your private key.
To OP's question, it depends where the country he is. But here our country we have a blockchain base wallet that may store fiat through remittance and you may convert bitcoin or ethereum in the option, so it is easy for us to buy and sell bitcoin here.