The only way to guarantee a sale within the quick sale market is to have total control. The difficulty is identifying the trustworthy companies as the industry has a very negative reputation.
There are over 25 companies in the UK that claim to buy properties quickly for cash; however very few actually buy properties, with the remaining ones being online estate agents pretending to be cash buyers.
They will tempt the seller offering a price at market value or slightly under with an aim to tie you into an option agreement for a period of 6 or 12 months at a fixed price. They will then market your property and attempt to find you a buyer rather than buying the property themselves. You will be tied into drawn-out processes and they may drop the offer price further down the line.