I'd also kind of be afraid to get robbed if I paid someone up front like that since I would not have much recourse under international law.
And there are plenty of these about, who prey on those wanting a work visa for the UK. There are always warnings on the work forums.
The sponsor pays for the visa and the applicant needs an interview, yet people report they paid thousands to someone who said they will get them a visa. Some even get given a CoS number but when they applied for the visa, they get deception on their record and a 10 year ban for using false documents to try to get a visa. Their CoS was either used and sold before (many times) or was not a real CoS.
Tier 2 visa holders are not allowed to work as contracters either, so beware of those give you a CoS for a visa and them farm you out to their clients.
In your position, I would be contacting the sponsor licence holders on that list I gave you and see if they can help you towards getting a Restricted CoS for a work visa.
TBH, if you aren't doing a job that is in demand in the UK, then you will struggle to get a sponsor as they can fill that post many times over from those who are allowed to work in the UK. Even though they can pay a foreign wokrer less money, UKVI won't issue the visa if it is a job they think could have been filled by someone already allowed to work in the UK: Brits, EEA citizens and their family members, someone on a spouse visa, a dependant of someone on a work or student visa, someone who just studied at a UK university as a Tier 4 visa holder.
The easiest visa to the the UK is the spouse visa. That visa allows you to work where you want to and do any job.