Hi.
It's not rude, but I'd be upfront about probably not going to buy it.
Has worked for me in the past.
First, kudos for showing Your age in your profile, not many teens tend to do that.
Nothing what so ever against You or the teens generally, but us older folks tend to be a bit grumpy towards younger folks.
Wait a few years and You'll be grumpy too.
Want to get there sooner, start a small business selling musical instruments

.
If it's any consolation, the music store owners/employees are a bit grumpy towards us older folks as well nowadays, after all, "we're there too just to try stuff out, and then buy it on-line".
They -rightfully IMO- feel like being the showroom for on-line retailers.
With a small business, it's all about building a trust and makig them look at You as a
customer. Instead of just as a someone who buys some random tidbit every once and a while, but wants to fondle expensive items often, in their view, just for the sake of fondling them no less.
I'd be upfront that I'm planning to buy a bass, possibly a Rickenbacker, or Sadowsky, or ... in a year or so, and possibly willing to buy it from them.
If they want to stay in business, and have You as their customer, they should at least consider everyone as a potential customer.
OTOH, be glad that You actually have an actual store to go to, not that many around anymore.
Good luck.
Regards
Sam