what his does is different than what you are trying to do. His creates a zone but it is not a "spawned entity" it is infact just some coords stored in varriables, that he uses to check if you are in this "zone". For your buyzone thing, the best thing to do is what I said first. Create a func_bomb_target, or info_bomb_target entity, and spawn it...then the game engine will automatically deal with them touching it and being able to plant ect.
[EDIT] Example:
Code:
new ent = engfunc(EngFunc_CreateNamedEntity, engfunc(EngFunc_AllocString, "func_bomb_target"));
//or is it info_bomb_target? try both...
if(ent) {
dllfunc(DLLFunc_Spawn, ent);
engfunc(EngFunc_SetOrigin, ent, Float:{1420.0, 705.0, 16.0});
engfunc(EngFunc_SetSize, ent, Float:{-128.0, -128.0, -64.0}, Float:{128.0, 128.0, 64.0});
} //change your coords to w/e
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