For a brief moment this morning, one of my shares had risen to a ridiculous price outside of trading hours. It dropped very quickly afterwards.
If I'd have put in an order to sell when it was at that high price, would that share price have to be honoured, or would the order only be properly placed during trading hours when the price was back to normal?
The listed price of a security isn't an objective fact, it's just the price at which the last transaction took place. Small cap shares are often subject to manipulation, particularly out of hours, so the listed price may not reflect the actual market. There's no guarantee that you'll get that price if you simply place a sell order - your broker will just sell at whatever price someone else is willing to pay at that moment.
Some brokers will process out-of-hours trades for some securities, but it depends on the trading platform. Pricing tends to be much more volatile outside of market hours and you may have to pay a higher commission. You could to place a limit order, which is an instruction to your broker to only sell at a specific minimum price. Conversely, you can also place a stop-loss order, which is an instruction to sell immediately if the price drops below a specified level. Another option is a fill-or-kill order, which says to your broker "sell if you can get this specific price, otherwise forget about it".
Another thing: the new "free" brokers (Trading212 and Freetrade at least) don't let you see a quote price for a buy/sell and then decide whether to execute. You have to just click buy and trust that the broker will follow all the rules and get the best price. In theory the broker is obligated by law to get the best price from the market makers, but I suspect there are loopholes in that law.
Brokers I have used that charge for trades (e.g. IWeb), do not have this problem.
Generally speaking, the "free" brokers don't actually trade on open markets, but via dark pools. These pools use the steady flow of retail trades (which are, for all practical purposes, totally random) to conceal large trades by institutional investors that might move the market. It's very slightly seedy, but it rarely disadvantages retail investors and it's the essential bit of financial magic that makes no-commission brokerage possible.
These retail brokers can't actually offer a quote price, because they can't see any of the orders on the other side - the whole point of a dark pool is that orders are secret until they've been fulfilled. They can almost guarantee that they'll fulfil orders at the best available price, but absolutely guaranteeing best prices might not be viable in very low liquidity conditions where there might be a substantial trade-off between price and execution time.
