As many of you know, the activities of the buying/selling of (Old School) RuneScape gold is not exactly ordained by Jagex without the usage of bonds only. Buyers will only ever receive a temporary ban if that at the billions mark, the risk is on the sellers. However, when it comes to the selling and buying of accounts, Jagex does absolutely nothing - but this market is absolutely difficult to navigate without getting scammed in the long-run due to account recoveries. Training skills can also result in accounts being hijacked and valuables stolen or botted on.
This is why middlemen exist in the community and have an actual purpose rather than making you feel safe. They do all the checks required to ensure a deal goes as it should and if not, they are held responsible.
On underground (Old School) RuneScape black-market communities such as Sythe, you will often see middlemen advertising their services as "free". These are people making money through tips, not actually through 100% generosity to the community. However, in some cases, these middlemen claim absolutely no responsibility/liability in case a deal goes sideways, so they may not even really cover you properly, if at all.
On such communities, there are typically middlemen who take a percentile fee and have ranks such as "OMM" - meaning official middleman. They have to be highly trusted in the community and vouched for to receive such a rank and generally they are the ones who can be trusted, just ensure to read their terms.
When you're dealing with RS middlemanning, they tend to conduct a lot of different checks on both parties, and may hold valuables. For example, holding gold whilst payment is sent to the seller - at which point the process is safe unless someone gets banned, if dealing with a safe payment method such as cryptocurrency. PayPal still has the ability to chargeback for any user. This includes background checks, has the account (on the underground community) been created a few days ago or a few years ago? An account created a few days ago is very high-risk. Are this person's vouches real or potentially fake and set up with alt accounts?
There are also lots of impostors in the underground community. Some people refuse to deal with Discord due to it. You see, Discord has a premium program called "Discord Nitro". It also allows invisible characters in someone's username (with the use of foreign characters) and with Discord Nitro, you can customize the number in your name. Thus, you could be John#0001 and someone else can also be called John#0001. Cons like these aren't possible with Skype and people do this via creating two Discord accounts (and servers) for the other party and middleman involved and con everyone by relaying messages, so even confirmation messages between real accounts on the forum communities seem genuine though they've both been talking to impostors. While any reputable community would make the middleman pay you back, likely even with 'no liability' clauses, as it was the middleman's explicit fault, if this scam was done against you only with no middleman, then you have no chance of being refunded.
A seasoned (Old School) RuneScape middleman would prevent this by using Discord developer IDs (each user no matter what nickname has an unique id), or through interacting with official Discord servers for that user where they simply have to click onto your name to speak to you.
Unfortunately, the sales of (Old School) RuneScape accounts can never be 100% safe with random people on the internet. RS middlemanning can help to an extent with background checks.
Even with recovery tests, the original account owner has all information available to recover you at any times, and you don't necessarily need all information to do a recovery, so some information may be withheld. And Jagex doesn't care about newer information such as recent password changes, they care about the original account owner and they can claim their account has been hijacked from many years ago essentially.
Selling accounts is best done with large businesses who will stick around. These people create accounts to actually sell. They aren't some random person who may feel like looting your account a few years down or want their account back after a few years as they regain interest in the game but don't want to grind all over again.
These large businesses will tend to stick around for the duration of (Old School) RuneScape. Also, with extra (insurance) fees in place, whether a middleman or a business, as the 1 in 10 case where a recovery occurs the fees accumulate for a refund.
Another tactic a middleman or business may employ is to also collect the seller's identification (and verify this is who they are paying to) and also may be able to escalate proof of an account sale to Jagex and get the account banned, further reducing incentive to recover. Using mules is also a semi-common practice, keeping wealth on completely unknown accounts in case your main gets compromised, not everything is gone. Odds are you aren't going to go through 1 billion GP in a day and Jagex will not ban the mules if operated to distribute wealth rather than real-world trade it as a seller. Many streamers do this.
First of all, a middleman needs to be trusted, you need to look for this person's vouches and how long they have been in the community, and their total traded amount. If they've traded $50,000 - there's minimal chance they are going to scam you for $50 as they will profit more in the long-run, unless it's an impostor or a liar. Ranks such as OMM on communities also mean they're more trusted.
Most importantly, look at the middleman's terms. If they hold no liability, that means they're only conducting some checks and may proceed with risky trades in order to get a tip. If anything goes wrong, they will not be held responsible and will not refund you if they explicitly claim no liability. This is why fee traders can potentially be better than free-middlemen who may claim much less responsibility, though even fee-traders may not claim responsibility for factors beyond their control.