I see no problems with 3m tape as long its Heavy Duty molding tape or automotive molding tape. My question is since you got it used is the tape still in good shape? if not go to autozone and re-tape it. Either take off the old tape or re-tape it on top of the old one.
Once you have done that you're ready to install it. Here is the most important thing, get a hairdryer or a heatgun. While you have your spoiler installed on your trunk heat up the spoiler around the edges where the tapes sit. This will help the adhesive on the tape to stick to the trunk a lot better.
Trust me I had to redo my roof spoiler because I installed it in a cold day. I didn't even think about using the hairdryer / heatgun to heat up the tape. But once I redid the whole thing with new 3m heavy duty / super strength automotive tape combining with the hairdryer. Thing is stuck like super glue, I can't even take it off I need KING KONG to remove it.
Hope this helps. Let us know what happens.
Oh i forgot to mention apply body pressure on the spoiler while heating it up with a hairdryer / heatgun