Is it your track record? Your team members' experience? Do you have testimonies from people who have bought the product? Does a popular person endorse it? Is it research backed? Do you have a money-back guarantee?
List them all.
Your offer is what they are getting in exchange for what you're selling to them.
Your offer is the PRICE + BONUS(ES) + MONEY BACK GUARANTEE + SCARCITY
It has to be BELIEVEABLE also.
Selling wristwatches? How about adding a bracelet as a bonus.
Female clothes? How about a pair of zirconia earrings. etc.
You get the idea.
Tell them they can return it if it's bad and get their money back too (depending on product/service type)
NOTE: People will NOT buy unless they think your offer is more valuable than the money that have to out up to get it.
Now writing the advert/marketing piece is a...
Hope this helped.