Hey mate,
Since you are selling a huge amount of items I would recommend doing them in smaller trades, this would minimize the risk of getting scammed. But if you are adamant on doing it all at once to minimized the risk of a ban on RuneScape then I suggest you trade with market leaders such as R2PLeasent and all the others people have suggested. Try to avoid payment systems such as PayPal as they can be trouble, even if your trading with a trusted person PayPal might limit your account for a whole range of different reasons and could find yourself with a 180 day wait to get your money. If you are from the UK then try out UK Bank Transfer, its where money goes direct to your bank account but if you are from a different country Western Union would be a safe bet, its cash in hand and once you have the cash then you can do what you like with it.
I believe h1j2q3 deals with Party Hats a lot so it would be a good idea to have a word with him as he may be able to offer you a price on all your items and I know he does offer Western Union and he is a experienced trader so he would be a safe bet to go with.
If you need anymore advice feel free to add my on Skype FendleSythe just make sure you trade with the correct people to avoid any problems.