Firstly, any roll or bag of coins can be manipulated. I have seen both occur; it does happen. However, you can make decisions that reduce your risk of being defrauded.
Rolls vs Bags of coins.
RAM rolls and security rolls are not invulnerable to tampering. I will not disclose the methods, but it is not difficult to make a roll look like it contains 25 2013 coronation coins when it may only have a single example. The fact is, if you buy coin rolls you will never know with 100% certainty what it contains until you open them. There is a caveat to this. If you buy directly from an authorised RAM distributor at release, you can be relatively confident that the roll does contain the specified coins.
How then can you go about buying rolls and bags of coins with confidence?
The best piece of advice I can give is to only buy from people you have a strong relationship with. If you deal with people you don’t know you're far more likely to get ripped off. If you deal with those with strong reputations you're likely to get the genuine article. Build relationships with people.
Don’t be afraid to ask questions. You can find out a lot about a potential seller by asking the right questions. If the coins are difficult to see, ask for clearer photos. Ask about the history of the roll. Where did the seller get it? How long have they had the item? Do they provide a money-back guarantee for the roll? How they answer these types of questions will give you some insight into the person you’re dealing with.
Assess what you are trying to buy. If you’re buying a 50 dollar roll of $2 coins for $75, there is now a particularly great incentive from people to manipulate the roll. The converse of this is true if someone is selling a roll of Coronations or Red Poppies at an extremely low price ask why are they doing this?
On eBay, I advise you to buy from sellers with strong feedback. Message the seller and ask them to guarantee that the roll/bag contains 25 of the same coin. Additionally, check that the seller has a money-back guarantee and a returns policy.
If you open the roll and it does not contain the specified items in most cases you will receive your money back once opening a case against the seller if they’re not responsive to your messages.
Just make sure to record yourself opening the package and the roll in one continuous video, this provides evidence of your claims. With a bit of common sense and following some of the tips above, you should be able to avoid purchasing rolls that are not genuine. If you are still unsure and want some advice, you can always call a coin dealer who may assist you.
Bags.
The benefits of bags are that you can see all the individual coins. Much of the analysis is removed for your purchase. It is not unknown for circulating coins to be put into a bag and then resealed and sold as uncirculated coins. Once again, assess the seller and make wise decisions about whom you purchase coins from.
Lastly, while bags may appear to be the best purchase, the coins are free to move around. Unless bags are taken care of well this can lead to more ‘bag-marks’ than coins from rolls. Often this will reduce the grade these coins will achieve and is another consideration when purchasing rolls or bags.