It’s not unusual for detectorists, especially those in clubs, to buy finds from each other. It cuts out any middle-men so can be advantageous to both parties.
Some detectorists are collectors and besides buying from their colleagues they also buy from dealers. Today I would strongly advise anyone buying coins and artefacts from dealers to ask for an unlimited guarantee of authenticity. There has always forgeries on the market but they have grown in number over recent years. Some very deceptive copies of Roman coins have been coming from Bulgaria, whilst a very wide range of English and foreign milled silver have been coming out of China. These copies are so convincing that I have no doubt that many will have been purchased as genuine and will now reside in private collections. Therefore, do take care when buying from sellers on some internets sites and places like car boot sales. Real bargains can occasionally be had from the latter but purchases often turn out to be cast copies and the seller might be difficult of impossible to trace.
Besides asking for a guarantee of authenticity, those buying from dealers should also ask for a guarantee that the stated grade of preservation is correct. If a coin is purchased as being in VF condition and it turns out to be only Fine then it would be worth considerably less. For example, if someone buys a VF Charles I half crown for £250 then if the grade is correct the price will be in line with the market; however, if it isn’t VF but only Fine then it would be worth not £250 but £60-70. Always remember that an over-graded coin is almost always an over-priced coin.