Is it legal to own?
Yes, it's legal for you to have Bitcoin at your own risk, the government or banks will not interfere on anything concerning that with you.
Do you have to declare it?
No, for the best of your own interests and to avoid any government agencies trouble like the EFCC, don't try to declare or make it public in Nigeria that you're into bitcoin, they may shift target on you for investigation on the source of your money and the rest by some of the government agencies.
Do you have to pay taxes on sale?
For now no, but they are making a bill to begin this same tax on citizens by 2026 but it's not clear if bitcoin users will also be included in this, but am sure of every citizen participation in paying as long as they are earning.
On which platforms are they purchased (are there physical stores where you can buy them?)
Unfortunately Binance and OKX exchange once allowed in Nigeria to operate p2p, but they had a serious challenge with the government concerning same tax related issues and were banned, but we still independently used some other exchanges like Bybit, MexC and Gate for our p2p as exchanges, for now, there are no stores or any related physical location where you can buy or sell bitcoin in Nigeria, despite the full engagement of the citizens in bitcoin, everyone tends to ne anonymous about it to avoid government threats.
What is the first use that comes to mind when you think of spending/using bitcoin?
Online exchange or trading.