No, it will not affect you. The above post is mostly correct, but it's misleading because it's from the perspective of a trader trying to make short term profits. But most of us are considered investors, we periodically buy/sell low volume of liquid ETF/funds/stocks and hold our positions for years. Market makers do collect a tiny premium for every trade, but it's irrelevant for the time horizon that investors are concerned with.

It depends on how thinly traded the stock is. Unless it's very thinly traded or you trade it after hours you'll almost certainly be completely fine. Like if you're talking about something heavily traded like a FAANG, you could probably dump several thousand shares pretty much any time during regular trading hours and have little or no effect on the price. Certainly not enough to be worth caring about for a one time transaction. OTOH if you're running some trading algo that does that kind of transaction thousands of times a day (or more) every day, then that would be a very different story.

Yes, it may! Assuming you are talking about at least a mid five figure position in a non MAG7 class sized stock, if you post the order directly to an exchange, that's enough to shuffle around the level 2 order book (obviously not in your favor).

By doing so you have completely identified yourself as non- informed, slow human flow. Ex: if you are looking to sell, it's blindingly obvious that the next likely move from you will be to lower the asking price. Even human traders will be a able to take advantage of that situation as a bread and butter trade.

One important aspect is that a lot of this is in terms of opportunity cost and risk. If you are posting the order at a "bad" time (let's say market makers are not long inventory and looking for liquidity), that's when one should expect front-running style action, as they want liquidity ahead of you. Likewise, if you're hanging out there and a market blip in the sector or in the depths of the market complex moves against you, you will get filled and "miss out" on the higher price that the price will settle at. And while you may think they don't care about a 50k order, these robots are hyper optimized and will have had PhDs and 9 figure plus data and infrastructure costs explicitly designed to capture every cent. That's why it's so obnoxious... It's not just market makers either. I know of a prop shop trade that involves harvesting rebates on trending stocks (stuffing the ask and amplifying the trend while receiving credits... if you've looked at stock charts you may have seen a seesaw pattern of liquidity exploration), and if you step into an active trade like that no doubt there will be at least some basic conditional logic to take advantage of stale liquidity.

I walk my very non-tech mother through manual executions on occasion. She finds it very funny that I can see her order, and without prompting has commented about how annoying the little game is.

Numbers - Let's say it's a 50 dollar stock that doesn't get a ton of volume. Most stocks are surprisingly illiquid. Which makes sense because of course nobody wants to deal with HFTs. The lit order book is almost a reference price for the actual trading that happens behind the scenes (midfills at dark venues, etc). Wouldn't surprise me at all to see 10 cents of additional slippage. That's $100. Also wouldn't shock me to see more if it's a smaller stock. Of course it's also fairly common for there to be midpoint liquidity right there for you to take. It just depends on the positioning of each of the participants, and a retail trader is at a distinct information disadvantage.

That said, it's highly unlikely that your 401k is at a DMA (direct market access) broker. Your order is probably first going to go to an internalizer (crossed with other customers), and then flashed to prop firms who will have the ability to take your order (and if they do it probably technically means that you've missed some money somewhere, although it may be in any number of obscure areas), and finally you're going to get sent to the market and pools of liquidity via a decent execution engine. On net, these routes don't work out that badly for retail participants.

Also, if you're talking a highly liquid ETF like S&P or Qs, don't worry about it. Just hit the bid.

That said, I would recommend upgrading if you can. Use a midpoint order type, split your order into chunks, spread it out time-wise a bit. Market On Open and Market On Close order types are also widely available at better retail brokers. I think that these are the most fair fills you can get. Split it 50/50 between open and closing auction. It's just a drop-down order type selector, and you can queue for the auction when you set up the order (say, early morning before the day starts) and walk away for the day and come back to filled orders.

Don't use market orders outside of the huge indexes and megacaps. You're guaranteed a bad fill, and then you also run the tiny risk of a truly awful fill (if something happens machine speed before you can blink... been there done that).

Market On Close / Market On Open orders are really easy to use. Brokers like Schwab and Fidelity and interactive brokers will support them. More people should use them. You'll be getting fair fills side by side with smart money.